Objective: Users of publicly funded voluntary HIV Counseling and Testing Centers (VCT C) in the Portuguese National Service have been found to have higher rates of HIV risk behavior than the general population, but data concerning the relationship between socio-demographic, behavioral, and cognitive variables, and Sexually Transmitted Infections (ST Is) in this specific population are limited. This study tests the impact of socio-demographic, behavioral and cognitive variables on ST Is in a sample of 168 users of VCT C. Methods: A cross-sectional study using structured questionnaires was conducted. Results: Being older, being gay or bisexual, and having more barriers towards safer sex (e.g., negative attitudes toward safer sex behavior) were associated with increased odds of having had a ST I diagnosis. We found significant differences between heterosexual and gay men for number of sexual partners, frequency of anal sex, exchanging sex for money or drugs, history of ST Is, AIDS knowledge, and risk beliefs about the partner, with gay men showing higher rates. Conclusions: ST Is education should target older, gay or bisexual individuals, and focus on information as well as on changing barriers towards safer sex and associated health beliefs.