Badie Aref, Aziz's lawyer, confirmed the report.
"His condition is serious and they will decide today whether he stays in hospital or should be returned to Camp Cropper," Aref told the AFP news agency, referring to a US-run prison where Aziz has been detained.
Lieutenant Colonel Pat Johnson, a spokesman for the US army in Iraq, said Aziz's condition was "improving but he is still being closely monitored".
Iraq's highest court sentenced Aziz and Ali Hassan al-Majid, a power-broker in Saddam Hussein's former government, to 15 years in jail last March.
The pair were convicted of crimes against humanity in regards to the killing of 42 Baghdad merchants in 1992.
Al-Majid and Aziz are also among 16 former officials on trial for a violent campaign against Shia Kurds.
They are accused of gassing members of the Fayli Kurdish community as part of a chemical weapons testing programme during Iraq's war with neighbouring Iran from 1980 to 1988.
Aziz surrendered to US forces in April 2003 after Saddam Hussein's government was overthrown.