The CIA has concluded that Cuban leader Fidel Castro is suffering from Parkinson's disease.
Castro continues to show strong stamina despite rumours about his health
The US agency is reported to have based its analysis on a variety of evidence, including observations of Mr Castro's public appearances.
Cuban officials declined to comment on the CIA report but insisted Mr Castro was in good condition.
The 79-year-old's health has long been the subject of rumours of illness, after almost half a century in power.
The CIA report suggests the disease has progressed far enough for US policymakers to raise questions about Cuba's future in the next few years.
"The assessment is that he has the disease and that his condition has progressed," said an unnamed official familiar with the report, quoted by Reuters news agency.
US diplomats however minimised the significance of the report, saying it would not influence policy decisions about Mr Castro or Cuba.
"We are not in any way adapting how we plan for the day Castro is gone based on an assessment that he might have Parkinson's," a state department official told Reuters.
Mr Castro has dismissed various reports of illness down the years as speculation spread by his enemies.
In a recent TV interview with former football star turned chat show host Diego Maradona, Mr Castro joked the rumours were so many that the day he died, nobody would believe it.
He has ruled for 46 years, making him the world's longest ruling head of government.
Observers say the leader's pace has slowed noticeably since he fractured his knee during a televised speech in December 2004.
But they say his stamina appears unabated, giving a series of almost daily hours-long talks on state television earlier this year.
Mr Castro's designated successor is his younger brother, 74-year-old Defence Minister Raul Castro.