Ron Reagan Saw Signs of Father's Alzheimer's at White House

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In an interview with "20/20", Ron Reagan, the son of President Ronald Reagan, said that when his father was in office, no one thought he had Alzheimer's disease -- the devastating illness he was diagnosed with less than two years after leaving the White House.

But Ron Reagan said that he did notice something had changed about his father during his presidency.The father and son often had spirited, fast-paced political debates when Ron visited the president at the White House. But during one such debate, Ron Reagan grew concerned.

"There was just something that was off. I couldn't quite put my finger on it," he told "20/20's" Elizabeth Vargas.

Watch Elizabeth Vargas' interview with Ron Reagan on "20/20" TONIGHT at 10 p.m. ET.

Ron Reagan grew worried enough to spend a day at the White House shadowing the president. That was when he noticed something else -- his father was reading note cards as he made phone calls. It wasn't an obvious sign that something was wrong, but it bothered Ron Reagan nonetheless.

"It wasn't like oh my God he doesn't remember he's President...You know, it was just -- I had an inkling something was going on," he said.

In hindsight, Ron Reagan believes what he say were early signs of Alzheimer's. But Reagan is adamant that his father's illness shouldn't mar his legacy.

"This no more discredits or defines his presidency than Lincoln's chronic depression, Roosevelt's polio, Kennedy's Addison Disease any of those things," he said. "You can't define their presidencies in terms of that."

To mark the hundredth anniversary of President Ronald Reagan's birth, son Ron Reagan has written "My Father at 100: A Memoir." Read an excerpt of the book below, CLICK HERE for photos of Ronald and Ron Reagan.

Excerpt: 'My Father at 100: A Memoir'

We all cobble together an internal account of our lives; in that, Dad was entirely typical. Virtually everyone creates a mental album of memories and anecdotes that, ultimately, passes for our version of a life story. We are all the protagonists of our own narratives, of course -- the indispensable main character; on a good day, the hero. In that sense, Dad was just like everyone else. Only peculiarly more so. For most of us, the boundaries of our personal tale are relatively fluid and amenable to outside influence. Our story selection, even the sense of our own character, shifts as new circumstances arise: One day we're a rebel folk hero in the making, the next day, a contented corporate cog. But Dad's story, I believe, was far more comprehensive in its sweep and consistent in its narrative details than is the case for most people. Keeping its primary themes intact and inviolate, safe from the depredations of an intrusive, ambiguous, and contradictory world, was for him an endeavor of existential import. My father didn't create his personal narrative to put one over on anyone. On the contrary, with its creation, he was forming a template for his life. He wanted to be seen -- he wanted to truly be -- an estimable individual who made his way through life as a positive force in the world, a man people would admire for all the right reasons.

Critics have long accused him of falseness, of merely acting out assigned roles. Such a superficial analysis ignores the central curiosity of my father's character: He played only one role, ever, and he did so unconsciously, totally absorbed in its performance.

Reading his early high school and college essays, and considering his film career, among other things -- and with plenty of opportunity for personal observation along the way -- I see two primary threads jumping out of my father's story line: that fierce desire to be recognized as someone noteworthy, even heroic; and his essentially solitary nature.

Ronald Reagan: 'The Inverse of an Iceberg'

On the one hand, he reveled in public exposure -- the bigger the stage, the more comfortable he was. He warmed to applause and the approval of crowds. He counted on the support of those around him. But in the film unwinding in his mind, Dad was always the loner, compassionate yet detached, who rides to the rescue in reel three. This role could become tedious -- it's fun to be bad on occasion -- and would prove unhelpful to his professional goals, for just as Hollywood was marketing ambivalent antiheroes, Dad was looking to wear an unblemished white hat in conventional westerns. Yet when he became a politician, circumstance and story line meshed beautifully: Don't we want our presidents to be heroes?

Ronald Reagan was the inverse of an iceberg: most of him -- the public man -- was plainly visible above the surface. Public Reagan sought glory on his college football team and when he broadcast sports events over the radio, acted in films, and entered the political arena with great success. He wanted and needed acclaim and recognition. At the same time, he would disavow ambition: It was crucial to his sense of self that he be seen working on behalf of others, and not for personal gain. But all the while, another, quieter Reagan, just as vital, rested invisibly beneath the waves. This hermetic self, who found outward expression mostly in the solitary acts of writing, ranch work, and swimming, was, in effect, the producer and director for the man onstage. In this Private Reagan, the personal drive he publicly forswore burned with a cold but steady flame. This private self, glimpsed only in fleeting, unguarded moments, formed his core. Without public acclaim, he may have been unfulfilled. Deprived of the opportunity to take refuge in his castle of solitude, he would have withered altogether. The Ronald Reagan with whom everyone is familiar could not have existed without the Ronald Reagan he rarely let anyone see.

Reprinted by arrangement with Viking, a member of Penguin Group (USA) Inc., from My Father at 100: A Memoir by Ron Reagan. Copyright © 2011 by Ron Reagan.