Doctors said Demjanjuk had a fever caused by an unidentified infection
The trial in Munich of alleged Nazi guard John Demjanjuk has been adjourned after the 89-year-old was taken sick.
The retired Ohio carworker had a fever that had continued to rise after medication, said the court, adding that his trial would resume on 21 December.
Mr Demjanjuk denies helping to murder 27,900 Jews at the Sobibor death camp.
His lawyer has called for the trial to be abandoned on grounds of poor health. Holocaust survivors have said Mr Demjanjuk is playing up his illness.
Prosecutors say the defendant volunteered to join the Nazis and shared their racist ideology.
The Ukraine-born accused maintains that he was a Soviet soldier who was captured by the Germans, and therefore spent most of the war in prison camps.
The trial is expected to last until May and, if found guilty, Mr Demjanjuk could be sentenced to 15 years in jail.
The court said Mr Demjanjuk had been examined by doctors in a prison hospital two hours before Wednesday's session was due to start, and found to have a fever caused by an unidentified infection.
"This chamber has determined not to proceed because it is not that the defendant does not want to come, but that he cannot come," said Presiding Judge Ralph Alt.
Wednesday's hearing was to have featured testimony from 40 relatives of victims of Sobibor - a camp in Nazi-occupied Poland.
A lawyer for some of the group, who are co-plaintiffs under German law, said they were "slightly frustrated" by the decision to postpone proceedings
Having been deported to Germany from the US in May, Mr Demjanjuk appeared weak during the first two days of his trial, lying in a wheelchair, covered in a blanket with his eyes closed.
Doctors had asked that daily hearings be limited to two 90-minute sessions.