Lawyer Badee Izzat Aref said he saw Tariq Aziz, who is awaiting trial for abuses under Saddam's government, on January 10 in detention in Baghdad.
He "looked very tired and was walking with the help of a soldier," Mr Aref said.
"He had also lost a lot of weight and told me he had suffered several slight heart attacks," Mr Aref said.
A US military official has denied Aziz's health had deteriorated.
"I cannot say what his pre-existing medical conditions are, but his health has not significantly diminished recently and there is no reason to believe that his health is going to deteriorate soon," said the official, who spoke on condition of anonymity as he was not authorised to talk to the media.
The official added Aziz was suffering from existing ailments when he was arrested after the overthrow of Saddam's regime in April 2003.
He is detained by US forces under agreement with the Iraqi government.
"He receives professional medical care of the same quality received by all our troops in Iraq, with a full-time medical staff dedicated to the care of those held at the detention facility," the official said.
Aziz's son Ziad, who lives in Jordan, said he believes his father's health is worsening, although he has not seen him recently.
He said his mother and sister Zainab left Amman today to go to Baghdad to see his father.
"I urge the Americans and the Iraqi government to release my father so that he can get the appropriate treatment," Ziad Aziz said.
At Christmas, US authorities allowed Aziz, a Christian, to speak on the phone for a few minutes to his wife and family in Jordan.
In November, Aziz's daughters Zainab and Shaymaa visited their father in Baghdad.
Mr Aref said he had spoken to the head of Aziz's detention facility about his client's health and urged him to do all he can to secure his release. He said he had also contacted the Iraqi Justice Ministry.