Former president Ronald Reagan, in a handwritten letter to "My fellow Americans" released yesterday in California, said that his doctors have told him he is suffering from the early stages of Alzheimer's disease, an incurable brain disease.

An accompanying statement by five of his doctors said the disease was detected during annual testing.

"Over the past 12 months, we began to notice from President Reagan's test results symptoms indicating the possibility of early stage Alzheimer's disease. Additional testing and an extensive observation over the past weeks have led us to conclude that President Reagan is entering the early stages of this disease," the doctors said.

The 83-year-old Reagan's health, they said, was "otherwise good," but "it is expected that as the years go on, it will begin to deteriorate."

The announcement yesterday came as no surprise to those close to the Reagans. Friends had noted that he was conspicuously absent last month during a conference at the Reagan Library in California and he had not appeared at various events in recent weeks. Several people who saw him in April at the funeral of former president Richard M. Nixon said at the time that he appeared to be in declining health.

Reagan's spokeswoman, Catherine Busch, said Reagan was at an undisclosed location yesterday with his wife, according to the Associated Press.

Alzheimer's disease is a progressive and irreversible neurological disorder. Symptoms include memory loss, impairment of judgment, disorientation and personality change. Reagan's mother, Nelle, who died at the age of 77, also apparently suffered from Alzheimer's.

During his presidency, Reagan at times seemed to have difficulty talking directly about some of his medical ills. In 1985, after he was operated on for colon cancer, Reagan never actually said he had the disease, but rather that "I had something inside of me that had cancer in it, and it was removed."

Two weeks later, an operation to remove a basal cell skin cancer from his nose touched off a battle with reporters at the White House because the administration did not report the operation until two days after it had occurred, and did not at first say that it involved skin cancer.

But in his letter yesterday, Reagan said he and his wife, Nancy, had decided to reveal the early diagnosis in the hopes of promoting a greater awareness of the disease.

"Unfortunately, as Alzheimer's Disease progresses," he said, "the family often bears a heavy burden. I only wish there was some way I could spare Nancy from this painful experience. When the time comes, I am confident that with your help she will face it with faith and courage."

Reagan said he intended to "live the remainder of the years God gives me on this earth doing the things I have always done. I will continue to share life's journey with my beloved Nancy and my family."

He thanked the American people for electing him president. "When the Lord calls me home, whenever that may be, I will leave with the greatest love for this country of ours and eternal optimism for its future. I now begin the journey that will lead me into the sunset of my life. I know that for America there will always be a bright dawn ahead."

Although at nearly 70 the oldest elected president, Reagan was always projected as energetic, with White House photographs showing him active at his ranch riding horses or chopping wood.

A former actor who was called the Great Communicator for his effective use of television, Reagan used his affability and sense of humor to deflect concerns during his reelection campaign that he was too old to be president.

"I will not make age an issue in this campaign," he said during a debate with Democratic candidate Walter F. Mondale. "I am not going to exploit for political purposes my opponent's youth and inexperience." The line brought down the house.

In his last physical examination as president in 1988, Reagan's White House physician said he was in "remarkable physical condition."

But Reagan had his share of ailments. He was severely nearsighted from childhood and he had been hard of hearing, especially in his right ear, after an actor fired a pistol near his head during the making of a movie.

He was severely wounded in a 1981 assassination attempt. A few months after his re-election, Reagan had two feet of his colon removed along with a cancerous growth. He underwent surgery for skin cancer three times. In July 1989, he was thrown from a horse and required an operation to remove a pool of blood from his brain.

President Clinton took time at a Democratic campaign rally in Oakland, Calif., to take note of Reagan's announcement. "It touched my heart in a particular way," he said, recalling a moment in his meeting with Reagan after the 1992 election in which the former president paused momentarily. " 'I forgot what I was talking about and it really makes me mad,' " Clinton quoted Reagan as saying.

"I want everyone of you in this room now to give Ronald Reagan a hand and wish him well and Godspeed," Clinton said.

Staff writers Lou Cannon and Ruth Marcus in California contributed to this report.