摘要Background The risk of HIV sexual transmission is much higher among people unaware of their HIV status than among those aware.Only a few studies have indicated that the incidence of unsafe sex can be reduced when people know their HIV status.This study was to investigate this effect in China.Methods A cohort study consisting of two surveys was conducted at two different times among a group of people living with HIV/AIDS,whose status was newly diagnosed with HIV via sexual contact,in Shanghai,Chongqing,and Kunming.The first survey was conducted among 823 people tested positive for HIV before notifying them of the HIV status.The second survey was conducted among 650 HIV-positive people at six months following the first survey (after notification of HIV status).The scope of survey covered unsafe sex practices,number of unsafe sexual partners,and frequency of unsafe sexual behaviors over the prior six months.Unsafe sex is defined as unprotected anal or vaginal sex with partners who are HIV positive or whose HIV status is unknown.Results The proportion of unsafe sex was reduced by about 85％ after HIV status notification.The risk of HIV sexual transmission was 15 times higher among persons unaware of their HIV status than among those aware.Approximately 95％ of new sexually transmitted HIV infections stemmed from 56％ of the infected persons unaware of their HIV status in China.Conclusion Timely HIV status notification has the potential to significantly reduce unsafe sex among HIV-infected persons and reduce the risk for HIV transmission via unsafe sex.