Men who have sex with men and women (MSMW) may expand the HIV epidemic from men who have sex with men to the female population. From a respondent-driven sampling survey in Shenzhen, China, we quantified the burden of HIV/syphilis and studied patterns of risk and prevention behaviors in 107 MSMW, and compared these with those of 542 men who have sex with men only (MSM-only). HIV prevention behaviors and consistent condom use with male partners did not differ between the two groups. However, HIV risk behaviors were more common among MSMW than MSM-only. Moreover, among MSMW, the HIV prevalence was as high as 6 % and consistent condom use was extremely low with female partners in MSMW. We conclude that there is risk of HIV transmission from MSMW to the female population. Special efforts are needed to convince MSMW they should refrain from HIV risk behaviors.