BACKGROUND: The objective of this study was to investigate risk factors of human immunodeficiency virus (HIV)-seropositive blood donors in Brazil and to determine if current donor deferral criteria are appropriate. STUDY DESIGN AND METHODS: Demographic and behavioral data among cases with confirmed HIV seropositivity (n = 272) were compared with those who had a false-positive serology (n = 468) between January 1999 and December 2003 in a case-control analysis with logistic regression. RESULTS: Risk factors that should have resulted in predonation deferral were reported by 48.9 percent of HIV-positive and 9.4 percent of false-positive donors. In multivariate analysis, male cases were significantly more likely to report male-male sex (adjusted odds ratio [AOR], 26.2; 95% confidence interval [CI], 7.8-87.4), a previous sexually transmitted disease diagnosis (AOR, 3.2; 95% CI, 1.5-6.9), exchanging money for sex (AOR, 2.1; 95% CI, 1.0-4.2), and at least two partners in the past 12 months (AOR, 2.3; 95% CI, 1.4-3.6). HIV-positive male donors were also more likely to be reactive for the presence of hepatitis C virus antibody (AOR, 4.0; 95% CI, 1.3-12.0) and hepatitis B virus core antibody (AOR, 3.8; 95% CI, 1.9-7.7). Female cases were more likely to report an intravenous drug user partner (AOR, 12.4; 95% CI, 1.3-120.2), a sexual partner with multiple sex partners or who had a history of sex with a sex worker (AOR, 13.0; 95% CI, 2.7-63.2), and at least two partners in the past 12 months (AOR, 2.2; 95% CI, 1.0-5.3). CONCLUSION: A substantial number of HIV-infected donors reported a risk factor that could have been identified in the predonation screening. Male-male sexual behavior was still the strongest determinant of HIV status in the studied population.