Consistent condom use is still a key component in the prevention of HIV/AIDS and other sexually transmitted infections. In this context, of particular interest are condom use patterns by medical students, especially considering the role of the latter as opinion-makers. The present article reports data on sexual behavior and its association with condom use among medical students in a private medical school in Rio de Janeiro State, Brazil. Data were collected from 643 medical students using an anonymous, self-administered questionnaire. Data analysis used Poisson regression with robust variance, calculating crude and adjusted prevalence ratios. Most students were sexually initiated (85.3%). The most common partners in their sexual debut were their boyfriends/girlfriends (55.1%), most often in their own homes (27.0%). Condoms were the main contraceptive method used at sexual debut (90.1%). In bivariate analysis, sex with a stable partner was the main factor associated with inconsistent condom use, and this association was confirmed by multivariate analyses, adjusted for age, gender, and family income (PR =1.95; 95%CI 1.47-2.57). These findings are consistent with the hypothesis that individuals with stable partnerships are less likely to perceive themselves at risk, following the supposition that the stability of the relationship protects against HIV/AIDS and other STIs.