Prison inmates are more vulnerable to HIV and other Sexually Transmitted Infections (STIs) due to risk factors such as needle sharing and unprotected sex with homosexuals. The aim of this work was to determine the seroprevalence and risk factors associated with the human immunodeficiency virus (HIV-1/2) and syphilis among male inmates in Caruaru, State of Pernambuco, Brazil. A cross-sectional study was performed between May and July 2011, when 1,097 inmates at a prison in Caruaru were assessed by means of interviews and blood sample collection for performing the respective tests. The prevalence was 1.19% for HIV infection and 3.92% for syphilis. HIV infection showed a statistically significant association (p <0.05), with injected drug use, homosexuality and blood transfusions. With respect to HIV status and syphilis, factors related to sex life were statistically significant (p <0.05). The prison population is a high risk group for the diseases investigated. The prevalence rates identified indicate the need to implement prevention programs, helping to contain such diseases in this particular population group.