The first medical evacuation from space remains a mystery as Soviet officials keep silent on the illness that forced three cosmonauts to return to Earth because the commander, Vladimir Vasyutin, 33, required hospitalization.

"We're still in the dark," a National Aeronautics and Space Administration flight surgeon who asked not to be identified said yesterday. "Some of our people had a meeting in Paris last week with a group of Soviet space medicine experts, who did not know themselves what happened to Vasyutin. We hope to be informed some time next month."

Vasyutin, Viktor Savinykh and Alexander Volkov flew back to Earth in a Soyuz spacecraft on Thursday after 65 days aboard the Salyut 9 space station. The Soviet news agency Tass said the flight was cut short when Vasyutin developed an ailment that required his hospitalization. Tass did not disclose Vasyutin's illness.

Officials at Houston's Johnson Space Center said that cosmonauts who stay in touch with U.S. astronauts have said Vasyutin caught a cold that grew worse and spread to his sinus cavity and lungs. The sources said Vasyutin had a fever that refused to break and may have come down with viral pneumonia, which cannot be treated with the antibiotics that cosmonauts and astronauts carry.

"He wasn't out-of-it sick, like he was delirious," one source said. "It's just that his illness could no longer be treated in orbit without the care a hospital can provide on Earth."

Another source said that amateur radio operators in Europe and the United States who listen to cosmonaut conversations began picking up encrypted air-to-ground conversations with flight directors in the Soviet Union in the last week of October, around the 40th day of the flight. This source said Radio Moscow stopped issuing daily bulletins on the crew at about the same time.

It was Vasyutin's first space flight, and Houston officials speculated that his immune system may have suffered from the stress of being in orbit. A form of white blood cell called the T-lymphocyte, which has a role in the body's resistance to illness, has been found to change in weightlessness.