This study evaluated differences in sexual behavior and risk for sexually transmitted diseases (STDs) among men who have sex with men (MSM) who met their partners on-line and those who did not. A self-administered questionnaire on sexual behavior was offered to a convenience sample of patients seeking public STD services. Thirty-two percent of MSM patients reported meeting a sexual partner over the Internet in the past year. MSM with on-line partners were younger, more likely to report sex with an HIV-positive person in the last year, and more likely to report casual partners in the last year compared with MSM with only off-line partners. HIV-negative MSM with on-line partners were more likely than HIV-negative MSM with only off-line partners to have received money or drugs for sex in the past year and to report sex with an HIV-positive partner in the past year. Although meeting partners on the Internet was common and associated with increased risk for STDs in MSM, it also presents new untapped opportunities for on-line health promotion and disease prevention.