VIENNA, Austria — What ails Viktor Yushchenko?

As Ukraine's popular pro-western opposition leader laid claim to victory Tuesday in hotly contested presidential elections, the mystery surrounding an appearance-altering illness that twice prompted him to check into a Vienna hospital persisted.

Yushchenko accused the Ukrainian authorities of poisoning him. His detractors suggested he'd eaten some bad sushi. Adding to the intrigue, the Austrian doctors who treated him have asked foreign experts to help determine if his symptoms may have been caused by toxins found in biological weapons.

Medical experts say they may never know for sure what befell Yushchenko. But the illness, whatever it was, has dramatically changed his appearance since he first sought treatment at Vienna's private Rudolfinerhaus clinic on Sept. 10.

Known for his ruggedly handsome, almost movie star looks, Yushchenko's complexion is now pockmarked and pasty. His face is haggard, swollen and partially paralysed. One eye often tears up.

Doctors at Rudolfinerhaus declined to comment Tuesday.

By the time Yushchenko checked out of the clinic last month after returning for follow-up treatment, physicians said they could neither prove nor rule out that he had been poisoned. Dr. Nikolai Korpan, who oversaw Yushchenko's treatment in Vienna, said the cause of his illness remained "totally open.''

Doctors were unable to confirm suspicions of poisoning because Yushchenko first checked into the clinic four days after the symptoms appeared -- too late for tests to show if poisoning had occurred, Korpan said.

At Rudolfinerhaus, Yushchenko underwent a week of intensive treatment for several illnesses, including acute pancreatitis, a viral skin disease and nerve paralysis on the left side of his face, Korpan said.

Clinic director Michael Zimpfer said doctors were unable to explain some of Yushchenko's symptoms, particularly his strong backaches. He said they could not rule out stress or a viral infection.

Yushchenko's doctors in Kyiv, the Ukrainian capital, said they had determined that "chemicals not of a food origin'' had sparked the illness.

Zimpfer and the clinic's chief physician, Dr. Lothar Wicke -- who requested police protection after receiving an anonymous threat while treating Yushchenko -- later asked for outside help from "a specialist in military operations and biological weapons,'' the Austria Press Agency reported.

Yushchenko's medical files have since been sealed and turned over to Austrian prosecutors, local media reported. Authorities have not said whether they planned to investigate further or merely turn over their findings to Ukraine.

Earlier this month, Volodymyr Syvkovych, the head of a 15-member Ukrainian parliamentary commission that investigated the mysterious illness, said a forensic medical examination found no traces of "any biological weapons'' in Yushchenko's blood, nails, hair or urine.

Yushchenko, meanwhile, has ridiculed the notion circulated by his political opponents in Ukraine that he simply ate a bad plate of sushi washed down with too much cognac.

"It was certainly no spoiled food,'' he said last month.