PRINCETON, N.J. — The simple fact was that he had done something wrong, and at the end of a long and revolutionary career it didn’t matter how often he’d been right, how powerful he once was, or what it would mean for his legacy.

Dr. Robert L. Spitzer, considered by some to be the father of modern psychiatry, lay awake at 4 o’clock on a recent morning knowing he had to do the one thing that comes least naturally to him.

He pushed himself up and staggered into the dark. His desk seemed impossibly far away; Dr. Spitzer, who turns 80 next week, suffers from Parkinson’s disease and has trouble walking, sitting, even holding his head upright.

The word he sometimes uses to describe these limitations — pathetic — is the same one that for decades he wielded like an ax to strike down dumb ideas, empty theorizing and junk studies.

Now here he was at his computer, ready to recant a study he had done himself, a poorly conceived 2003 investigation that supported the use of so-called reparative therapy to “cure” homosexuality for people strongly motivated to change.

What to say? The issue of gay marriage was rocking national politics yet again. The California State Legislature was debating a bill to ban the therapy outright as being dangerous. A magazine writer who had been through the therapy as a teenager recently visited his house, to explain how miserably disorienting the experience was.

And he would later learn that a World Health Organization report, released on Thursday, calls the therapy “a serious threat to the health and well-being — even the lives — of affected people.”

Dr. Spitzer’s fingers jerked over the keys, unreliably, as if choking on the words. And then it was done: a short letter to be published this month, in the same journal where the original study appeared.

“I believe,” it concludes, “I owe the gay community an apology.”

Disturber of the Peace

The idea to study reparative therapy at all was pure Spitzer, say those who know him, an effort to stick a finger in the eye of an orthodoxy that he himself had helped establish.

In the late 1990s as today, the psychiatric establishment considered the therapy to be a nonstarter. Few therapists thought of homosexuality as a disorder.

It was not always so. Up into the 1970s, the field’s diagnostic manual classified homosexuality as an illness, calling it a “sociopathic personality disturbance.” Many therapists offered treatment, including Freudian analysts who dominated the field at the time.

Advocates for gay people objected furiously, and in 1970, one year after the landmark Stonewall protests to stop police raids at a New York bar, a team of gay rights protesters heckled a meeting of behavioral therapists in New York to discuss the topic. The meeting broke up, but not before a young Columbia University professor sat down with the protesters to hear their case.

“I’ve always been drawn to controversy, and what I was hearing made sense,” said Dr. Spitzer, in an interview at his Princeton home last week. “And I began to think, well, if it is a mental disorder, then what makes it one?”

He compared homosexuality with other conditions defined as disorders, like depression and alcohol dependence, and saw immediately that the latter caused marked distress or impairment, while homosexuality often did not.

He also saw an opportunity to do something about it. Dr. Spitzer was then a junior member of on an American Psychiatric Association committee helping to rewrite the field’s diagnostic manual, and he promptly organized a symposium to discuss the place of homosexuality.

That kicked off a series of bitter debates, pitting Dr. Spitzer against a pair of influential senior psychiatrists who would not budge. In the end, the psychiatric association in 1973 sided with Dr. Spitzer, deciding to drop homosexuality from its manual and replace it with his alternative, “sexual orientation disturbance,” to identify people whose sexual orientation, gay or straight, caused them distress.

The arcane language notwithstanding, homosexuality was no longer a “disorder.” Dr. Spitzer achieved a civil rights breakthrough in record time.

“I wouldn’t say that Robert Spitzer became a household name among the broader gay movement, but the declassification of homosexuality was widely celebrated as a victory,” said Ronald Bayer of the Center for the History and Ethics of Public Health at Columbia. “ ‘Sick No More’ was a headline in some gay newspapers.”

Partly as a result, Dr. Spitzer took charge of the task of updating the diagnostic manual. Together with a colleague, Dr. Janet Williams, now his wife, he set to work. To an extent that is still not widely appreciated, his thinking about this one issue — homosexuality — drove a broader reconsideration of what mental illness is, of where to draw the line between normal and not.

The new manual, a 567-page doorstop released in 1980, became an unlikely best seller, here and abroad. It instantly set the standard for future psychiatry manuals, and elevated its principal architect, then nearing 50, to the pinnacle of his field.

He was the keeper of the book, part headmaster, part ambassador, and part ornery cleric, growling over the phone at scientists, journalists, or policy makers he thought were out of order. He took to the role as if born to it, colleagues say, helping to bring order to a historically chaotic corner of science.

But power was its own kind of confinement. Dr. Spitzer could still disturb the peace, all right, but no longer from the flanks, as a rebel. Now he was the establishment. And in the late 1990s, friends say, he remained restless as ever, eager to challenge common assumptions.

That’s when he ran into another group of protesters, at the psychiatric association’s annual meeting in 1999: self-described ex-gays. Like the homosexual protesters in 1973, they too were outraged that psychiatry was denying their experience — and any therapy that might help.

Reparative Therapy

Reparative therapy, sometimes called “sexual reorientation” or “conversion” therapy, is rooted in Freud’s idea that people are born bisexual and can move along a continuum from one end to the other. Some therapists never let go of the theory, and one of Dr. Spitzer’s main rivals in the 1973 debate, Dr. Charles W. Socarides, founded an organization called the National Association for Research and Therapy of Homosexuality, or Narth, in Southern California, to promote it.

By 1998, Narth had formed alliances with socially conservative advocacy groups and together they began an aggressive campaign, taking out full-page ads in major newspaper trumpeting success stories.

“People with a shared worldview basically came together and created their own set of experts to offer alternative policy views,” said Dr. Jack Drescher, a psychiatrist in New York and co-editor of “Ex-Gay Research: Analyzing the Spitzer Study and Its Relation to Science, Religion, Politics, and Culture.”

To Dr. Spitzer, the scientific question was at least worth asking: What was the effect of the therapy, if any? Previous studies had been biased and inconclusive. “People at the time did say to me, ‘Bob, you’re messing with your career, don’t do it,’ ” Dr. Spitzer said. “But I just didn’t feel vulnerable.”

He recruited 200 men and women, from the centers that were performing the therapy, including Exodus International, based in Florida, and Narth. He interviewed each in depth over the phone, asking about their sexual urges, feelings and behaviors before and after having the therapy, rating the answers on a scale.

He then compared the scores on this questionnaire, before and after therapy. “The majority of participants gave reports of change from a predominantly or exclusively homosexual orientation before therapy to a predominantly or exclusively heterosexual orientation in the past year,” his paper concluded.

The study — presented at a psychiatry meeting in 2001, before publication — immediately created a sensation, and ex-gay groups seized on it as solid evidence for their case. This was Dr. Spitzer, after all, the man who single-handedly removed homosexuality from the manual of mental disorders. No one could accuse him of bias.

But gay leaders accused him of betrayal, and they had their reasons.

The study had serious problems. It was based on what people remembered feeling years before — an often fuzzy record. It included some ex-gay advocates, who were politically active. And it did not test any particular therapy; only half of the participants engaged with a therapist at all, while the others worked with pastoral counselors, or in independent Bible study.