A study of sexual behavior in migrant men was conducted in construction sites, markets, and factories in Shanghai, the largest city in China. An anonymous, self-administered questionnaire was completed by the migrants. Among 986 sexually active men, 14% had had more than one sexual partner in their lifetime, 31% premarital sex, 3.3% oral sex, and 11.5% commercial sex. Seventy-eight percent never used condoms. Risk-taking was similar in different working venues but increased with decreasing age. Younger men initiated sexual intercourse earlier and had more premarital sex and sexual partners as well as more frequent sexual encounters. Risk-factors that correlate with having multiple sex partners included having seen pornography, early age at initiating sexual intercourse, premarital sex, and a desire for legalization of commercial sex. Sexually transmitted diseases and HIV are likely to increase among migrants, particularly those who are young. Intervention strategies should target younger migrants, and must accommodate a low literacy level. (Contains 3 tables and 1 figure.)