Eighty-two male transvestites imprisoned in Casa de Detenção (São Paulo, Brazil) were tested for HIV antibodies, and completed a questionnaire investigating their demographics, arrest and imprisonment records, sexual practices, and drug use. Data were then analyzed to evaluate the incidence of HIV infection and its association with various behavioral and other factors. Sixty-four of 82 (78%, 95% confidence interval [CI], 67-87%) transvestites were positive for HIV infection. The factors associated with significant differences in positivity among these individuals were the time spent in prison and the number of sexual partners during the previous year. It appears that the high rate of infection in this group obscured the importance of other risk factors and behavioral patterns potentially associated with infection. Given the social environment and the high rate of HIV infection among imprisoned transvestites, their role as "vectors" for dissemination of HIV in urban areas of Brazil may be significant.