Mr. Reagan ''absolutely'' did not ''show any signs of dementia or Alzheimer's,'' said Dr. John E. Hutton Jr., who cared for him from 1984 until the end of his Presidency and remains a close family friend. Extensive mental-status tests did not indicate evidence of Alzheimer's until 1993, more than four years after Mr. Reagan left office, Dr. Hutton said.

Even in hindsight, Mr. Reagan's friends and former aides said that they, too, had seen no hint of the deterioration to come. And while they acknowledged that he had occasional memory lapses as President, especially when it came to names, many said he had had these problems for years, certainly since he was Governor of California, from 1967 to 1974.

Mr. Reagan is thought to be the first President or former President to have Alzheimer's. But the disease -- a form of dementia, or senility, that strikes with increasing frequency as people advance beyond their 60's -- is a growing public health problem in an aging society. While the course of Alzheimer's varies, it is often slow, measured in years; as it advances, abnormal deposits of protein destroy the nerve cells in the brain, obliterating memory. The two approved drugs can do no more than stave off decline for a few months, and only for some people. Ultimately, Alzheimer's is fatal, though many people with the disease die of other causes.

The first significant hints that Mr. Reagan was crossing that fuzzy line into dementia, his doctors said, did not come until September 1992, three years and eight months after he left office. From that point on, they described a gradual descent into bewilderment and forgetting that will be achingly familiar to families and friends of the four million Americans who share his fate.

The Decline

Mental Lapses Warn Of Worse to Come

On Sept. 13, 1992, Mr. Reagan made a campaign speech for President George Bush in Yorba Linda, Calif. In suit and tie on that sweltering day, but speaking more slowly than in the past, Mr. Reagan drew thunderous cheers from the shirt-sleeved crowd.

Dr. Lawrence C. Mohr, one of the White House doctors in Mr. Reagan's second term, was seeing him for the first time in six months, and afterward, the doctor and the former President talked. As usual, Mr. Reagan asked about Dr. Mohr's family. But Mr. Reagan ''was distant,'' he said, and seemed ''preoccupied, which was unusual, because Ronald Reagan is a person who was engaged when he would talk to you.''

At the end of the conversation, the doctor continued, ''Mr. Reagan asked me, 'What am I supposed to do next?' There was a blank look on his face.'' Dr. Mohr said he guided Mr. Reagan away and wondered ''what had caused the change and what was going to happen.''