EFF officials who went to visit AbaThembu King Buyelekhaya Dalindyebo in an East London prison on Saturday were informed he was in hospital, the party said.
"We don't know what is wrong and were advised we will receive info when we are there," EFF national spokesperson Mbuyiseni Ndlozi told News24.
He said Dalindyebo was apparently admitted to St Dominic's Hospital on Friday night.
The Economic Freedom Fighters delegation, led by leader Julius Malema, had arrived at the East London Maximum Correctional Centre just after midday for a courtesy visit.
They would now move to the hospital, which is about 6km from the prison.
After unsuccessfully trying several legal avenues to avoid jail, Dalindyebo started serving his 12-year sentence last month.
In 2009, Dalindyebo was sentenced to 15 years in prison for culpable homicide, assault with intent to do grievous bodily harm, arson, and kidnapping.
He was granted bail pending the outcome of his appeal to the Supreme Court of Appeal.
In October last year, the SCA set aside his culpable homicide conviction and reduced his sentence to 12 years.
His crimes involved the treatment he meted out to some of his subjects in 1995 and 1996.