BACKGROUND Unprotected anal sex (insertive or receptive) is an important risk factor for HIV infection and other STDs. OBJECTIVE The objective was to investigate individual and social factors associated with unprotected anal sex among Men Who have Sex with Men (MSM) that declared being HIV negative and engaged in anal sex with an HIV positive partner or of ignored serology. METHODS This study is part of a survey among MSM and among Internet users, with data collected from an on line questionnaire in project site. The internet sample for this study was 487 MSM. Descriptive and stratified analyses were carried out and χ² test was used. Logistic regression was used for modeling the data. RESULTS Among the participants 30.8% referred unprotected anal sex in the previous 12 months with an HIV positive partner or partner with unknown serology. Most of them belonged to higher social classes, were white and single. Factors associated with unprotected anal sex were: number of partners (more than 3) in the previous 12 months (OR=1.7; 95% CI 1.1-2.7), high or medium perception of risk to HIV/Aids (OR=2.7; 95% CI 1.5-3.6) and enjoy passive oral sex (OR=0.3; 95% CI 0.2-0.8). CONCLUSIONS Unprotected anal sex was a frequent practice among MSM in this survey. The frequency to public bathrooms should be further explored as a risk factor for unprotected anal sex. It is important to further explore these findings in order to understand these relationships and the involvement in risky sexual practices for HIV infection among MSM.