A prominent Chinese Aids campaigner, Wan Yanhai, has gone missing after being questioned by police in the capital, Beijing, his office has said.
Rights groups have accused Beijing of harassing Aids activists
Mr Wan has not been heard from since Friday, his advocacy group Aizhi, said in a statement.
Mr Wan is one of China's best-known Aids campaigners and has criticised the government's response to the spread of the disease.
In 2002 he was held for three weeks and accused of disclosing state secrets.
Aizhi said police questioned Mr Wan at its offices in Beijing on Friday.
He ordered workers to cancel plans for an Aids symposium scheduled for Sunday, and has not been heard of since a brief telephone call to a colleague on Friday night.
"The colleague asked Wan Yanhai his whereabouts, and Wan Yanhai replied that he was being questioned," the statement said.
"Since then, his colleagues and family have lost contact with Wan Yanhai."
The group said Wan Yanhai had told colleagues on Friday to ensure than any participants who had come to Beijing for the symposium returned home.
Human rights groups have accused the Chinese government of harassing Aids activists working in the country.
In the past couple of years, Beijing has taken a more open approach to the fight against Aids after years of denying there was a problem.
China is thought to have about one million people with HIV.
The United Nations says that without immediate action to educate the public, China could have 10 million people with HIV by the end of the decade.