Incarcerated women as a group are particularly vulnerable to infections. The lack of public programs for prevention, early diagnosis, and treatment contribute to the increase in the incidence and prevalence of diseases in general and especially sexually transmitted diseases. This article aims to estimate the prevalence of infection by the human immunodeficiency virus (HIV), human papillomavirus (HPV), and syphilis among inmates at the Women's Penitentiary in the State capital of São Paulo, Brazil. All inmates were invited to participate in the study, which was divided into two stages: 1. STD/AIDS preventive workshops including interviews and 2. laboratory tests. The interview covered knowledge of STD/AIDS, risk behavior, and individual reproductive health history. A total of 262 women, with a mean age of 32.4 years and limited schooling, participated in more than one stage of the study. Prevalence rates were 14.5% for HIV, 16.3% for high-oncogenic-risk HPV probes, 4.8% for low-oncogenic-risk HPV probes, and 5.7% for syphilis. The authors conclude that STD/HIV constitute a serious health problem in the prison system, requiring urgent preventive measures.