Objective: To verify the prison population's knowledge of sexually transmitted infections. Methods: An illustrated questionnaire was prepared based on the Ministry of Health's information booklet to provide information about sexually transmitted infections and applied to a sample of the prison population (PP) (n=158). This is a descriptive and cross-sectional research conducted in 2017 in one of the penitentiaries in the western region of the state of São Paulo. Results: The study sample

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... ed both correct knowledge and misconceptions regarding the identification, prevention, treatment and ways of transmission of these infections. Among these aspects, the identification of these infections raised the most doubts, especially about those that produce penile lesions. Conclusion: The data obtained in the study reinforces the need for greater investments in the knowledge of sexually transmitted infections in the prison setting. They should focus on educational activities contextualized to the profile of this population: young adults with low education and little access to information.