Polanski, 76, was arrested Sept. 26 in Zurich in connection with a 1977 child-sex case in Los Angeles.
The famous filmmaker, who lives in France, has not traveled to the United States since he pleaded guilty in a Los Angeles court to having unlawful sex with a minor and then fled the country before he could be sentenced. He was accused of drugging and raping Samantha Geimer when she was 13.
Polanski's lawyers have alleged misconduct on the part of the Los Angeles criminal justice system at the time of the plea deal. They are fighting extradition from Switzerland to the United States.
People.com said Monday Polanski had been transferred from prison to a hospital in Zurich Friday to undergo tests for an undisclosed medical condition. He reportedly was expected to stay at the facility for several days.
The magazine's Web site said the ailment is a pre-existing condition that cannot be treated while Polanski is behind bars.
The Swiss newspaper Blick said Polanski was also admitted to the hospital for X-rays on a previous occasion since his arrest.
"His general condition is no longer satisfactory," People.com quoted Polanski's lawyer, Herve Temime, as saying in a radio interview. "But the situation isn't easy for any prisoners, even less so for a man the age of Polanski. One has to be attentive."