To describe the characteristics of cases of syphilis amongst prison inmates. Descriptive study. Confirmed cases of primary, secondary and early latent syphilis were identified in prisons in Spain during 2007-2008. Socio-demographic and clinical information, as well as variables related to transmission, was collected by the attending physicians in a standard form. Frequency distributions of each variable were performed. Annual incidence rates were calculated. To evaluate the association between qualitative variables, the χ² and Fisher's exact tests were used; the Mann-Whitney test was utilized to compare quantitative variables. During the study period, 94 syphilis cases were identified (35.1% primary, 20.2% secondary and 44.7% early latent). The incidence rates were 0.9 cases/1000 prisoners in 2007 and 0.7 cases/1000 prisoners in 2008. Most cases were male (90.4%), between 31-40 years old (30.9%) and foreigners (52.1%). The majority of patients were diagnosed through screening (80.9%). Heterosexual contact was the most frequent transmission route (83.0%). Overall HIV prevalence was 5.3%, and 16.0% of the patients had a history of previous sexually transmitted infections (STI). Almost 40% of the cases reported being a client of a sex worker. Incidence of syphilis in prison is high. Many syphilis patients were detected through screening, highlighting the role of the Spanish prison health service in STI control.