WASHINGTON – The Cuban dictator Fidel Castro is suffering from Parkinson’s disease, which may soon loosen his grip on the communist state, the CIA believes.

Briefings given to Congress members by America’s main spy agency claimed the 79-year-old has had the illness for some time but it has recently worsened.

The conclusion is based on film of Mr. Castro’s public appearances and the opinions of CIA doctors. Last year the revolutionary leader fell over after a speech, breaking an arm and a knee, but he has since made other lengthy speeches. Any hint that the long-expected fall of Mr. Castro is about to occur will send a shiver of delight through Washington, which has used every possible means to remove him from power for more than four decades.

While a post-Castro Cuba might peacefully return to the democratic fold, it could also descend into chaos. That would present Washington with a series of dilemmas. The most pressing of which would be the question of a replacement for Mr. Castro, who seized power in 1959. Under the communist party succession, the dictator plans to hand power to his brother Raoul, 74.

Washington’s hopes rest with the anti-Castro opposition, but it is dispirited, divided, and harassed by the Cuban secret police.