China's state media says unsafe sex has, for the first time, become the main means of transmission of HIV/Aids, overtaking intravenous drug use.
Infected blood transfusions also caused many of the early cases.
The news raises fresh concerns that HIV infections are moving from high risk groups to the mainstream population.
China estimates that about 650,000 people are HIV positive - but it is thought that widespread under-reporting makes accurate figures hard to come by.
An official report says China saw 70,000 new cases of HIV in 2005 - and almost half were contracted from sexual intercourse.
This has profound implications for China's approach to HIV prevention.
In the past, the main focus has been on specific high-risk groups. Now efforts will have to concentrate on addressing mainstream sexual behaviour.
That is likely to be embarrassing in a society which is still deeply conservative about sex.
Despite the fact that many people in China find these issues so difficult to talk about, sexual behaviour is changing, partly because of China's increased exposure to outside influences, and partly related to the millions of mostly male migrant workers living in the cities.
Many of these workers are separated from their wives and families.
Some surveys suggest one in 10 sexually active men in China today has bought sex from a prostitute.
Even if sex workers know about HIV/Aids, it can be difficult for them to insist that clients use condoms.
Pre-marital sex is also becoming more acceptable. The Chinese government is struggling to adopt the level of openness it needs to tackle the consequences of these changes in behaviour - especially the impact on health.
Prevention programmes tend to be small-scale, and although non-government groups and campaigns are allowed, they are often viewed with suspicion and carefully controlled.
The high degree of stigma associated with HIV - and a lack of confidentiality - can also deter people from being tested at all.