A court order issued Monday noted that 89-year-old Johann Breyer had been hospitalized but didn't cite his condition.
Breyer's lawyers have said he's in frail health, but a phone message seeking comment from one of them wasn't immediately returned Monday.
Breyer, a retired toolmaker, has been in federal custody since his arrest last month at his longtime home in Philadelphia.
German authorities have said they hope to try him on accessory-to-murder charges for his guard service at the Auschwitz death camp in 1944. He has denied taking part in the mass killing of Jews and others there.
U.S. Magistrate Judge Timothy Rice cancelled an extradition hearing scheduled for Thursday, citing Breyer's hospitalization. He said he'll make a decision based on written arguments.
German authorities in the Bavarian town of Weiden issued a warrant for Breyer's arrest in June 2013. The warrant accuses Breyer of 158 counts of accessory to murder one count for each trainload of victims taken to the Auschwitz death camp in occupied Poland from May 1944 to October 1944.
Breyer said in 2012 that while he was a guard at Auschwitz he was assigned to a part of the camp that was not involved in the slaughter of Jews and others.
"I didn't do anything wrong," he said.
Breyer moved to Philadelphia after World War II and for decades had lived a quiet, middle-class life. He has American citizenship because his mother was born in the U.S.