According to his publicist, David O'Neill, Keillor was treated in Washington D.C. before doing shows this past Friday and Saturday.
In his posting, Keillor said he flew to the Mayo Clinic for further testing.
"Saw an MRI image of my skull with a black hole where a previous stroke struck close to the language center of the brain, so I came away feeling vastly fortunate," he wrote.
Keillor said he is on a anti-convulsant that makes him feel lethargic, but "I intend to fight this."
"I have five more shows and want them to be terrific," said Keillor who will depart the show he created in early July. "Plenty of time for lethargy in August."