Presiding Judge Ralph Alt said the doctor examined Demjanjuk, 89, in a prison hospital two hours before the session due to begin, and determined that he had a fever caused by an unidentified infection. The fever continued to rise despite medication, and the doctor decided it was not safe to transport him to court, Alt said.
"This chamber has determined not to proceed because it is not that the defendant does not want to come, but that he cannot come," Alt said.
The day was to have featured more testimony, which began Tuesday, from some of the approximately 40 relatives of victims who have joined the trial as co-plaintiffs as allowed under the German legal system.
Michael Koch, an attorney for about 30 of them, said his clients were "slightly frustrated" by the decision, but had always understood it was a possibility and would tell their stories another day.
"All in all, there were two good days," he said in reference to the first two days of the trial on Monday and Tuesday.
Demjanjuk was deported in May from the United States to Munich, and has been in custody since then. He could face up to 15 years in prison if convicted for his alleged activities training as a guard in the SS camp Trawniki, then serving in the Sobibor death camp in occupied Poland.
The prosecution argues that after Demjanjuk, a Soviet Red Army soldier, was captured by the Germans in 1942 he volunteered to serve under the SS as a guard.
Demjanjuk denies ever having served as a guard, saying that he spent most of the rest of the war in Nazi POW camps before joining the so-called Vlasov Army of anti-communist Soviet POWs and others. That army was formed to fight with the Germans against the encroaching Soviets in the final months of the war.
Demjanjuk appeared feeble during the first two days of his trial, lying on a gurney covered in a blanket with his eyes closed during the proceedings - though opening his eyes and talking with his attorneys and others during breaks.
Sobibor survivor Thomas Blatt, who does not remember Demjanjuk himself at the camp but is to testify in January in general about his experiences, said he had expected trial sessions to be canceled.
"It's tiring to lie down a whole day in front of so many people, so I guess he wanted a break," Blatt said cynically.
Demjanjuk's attorney Ulrich Busch filed a written motion Wednesday to have the case against him thrown out, arguing that Polish investigators in 2007, working from the same evidence, determined there was not enough proof to proceed to trial. Busch said he argued in the motion that European Union rules prohibit the German investigation from going forward without any new evidence if prosecutors had elsewhere in the bloc had decided to drop it.
Alt told The AP he did not know when the court would rule on the motion, but suggested it could make a decision before the trial is scheduled to resume Dec. 21.
Similar motions filed by Busch ahead of the trial have been rejected.