Ratko Mladic is in a prison hospital in The Hague it emerged last night.
However, he will still go before the war crimes tribunal for the former Yugoslavia today to face charges of genocide, his court-appointed lawyer said.
Aleksandar Aleksic, a prominent Belgrade lawyer, said he had met the 69-year-old former Bosnian-Serb army commander in a hospital room set aside for defendants.
He said the health of the man, who is now the tribunal's biggest case, had deteriorated because of years of neglect while living as a fugitive.
"It is the tribunal's regular procedure to carry out medical examinations, including tests, during the first days of arrival of an accused at the detention unit.
"The same procedure applies to Mladic. So there is absolutely nothing unusual in the fact that Mladic is held at the medical facility of the prison where those tests can be performed," the tribunal said in a statement.
Mr Aleksic said Mladic "has not had proper health care for years and his condition is not good".
As reported in the Serbian media following his capture last week, Mladic has partially lost the use of one hand due to a stroke suffered years ago.
But Mr Aleksic said he seemed mentally capable and responsive.
Mladic is due at the tribunal today to face charges of genocide, resulting from his role in the 1995 Srebrenica massacre of more than 8,000 Muslim males and for the 43-month siege of Sarajevo, from 1992 to 1995, in which 12,000 were killed.
"He will be transferred to the detention unit tomorrow. He is in a prison hospital, which has two rooms dedicated to UN detainees," Mr Aleksic said.
"He has a room to himself, with a small outdoor yard where he can walk and has been making phone calls to his family.
"I am going to ask tomorrow that he be given additional medical tests," he added.
Mladic will have an opportunity at today's hearing to address his health issues and the conditions in in which he is being detained.
Serbian media reports say he is unlikely to enter a plea today as, under the rules of the war crimes tribunal, he can defer that step for 30 days.