Mr Reagan has been out of the public eye since November, when he said he had the neurodegenerative disorder. Recent reports of his condition have conflicted. A former staff member, Frederick J Ryan, said that when he saw Mr Reagan in mid-May he had no problem recalling his years as President.
However, Nancy Reagan has told friends that her husband cannot always recall having lived at the White House and, at a dinner in his honour recently, he failed to comprehend that the applause was for him.
In a speech earlier this month she said that Alzheimer's "is a really very cruel disease".
Despite his periodic forgetfulness, Mr Reagan is unlikely to be found wandering the streets near his Bel-Air home - he's constantly under the watch of the Secret Service.
According to his aides he visits his Santa Barbara mountain-top ranch where he chops wood, plays golf, and goes to his Los Angeles office five days a week, where he meets visitors and signs books and photographs.
The secrecy surrounding his affliction is in keeping with his family's wish. "The Reagans want Americans to remember the President as vigorous rather than in deterioration," said a source close to the family.
"The most important thing people should know is that he has this wonderful serenity. He's doing well," said his daughter, Patti Davis, in a recent interview.
In his letter "To my fellow American's" last year, Mr Reagan was applauded for his frankness. "I now begin the journey that will lead me into the sunset of my life," he wrote.
Alzheimer's effects an estimated 4 million people in the US. Last month Mrs Reagan accepted an award from the Harvard Mahoney Neuroscience Institute for helping to increase public awareness of it.