OBJECTIVES: To describe the use of primary care services by a prisoner population so as to understand the great number of demands and therefore to plan services oriented to the specific needs of these patients. DESIGN: Retrospective cohort study of a sample of prisoners' medical records. SETTING: All Belgian prisons (n = 33). PATIENTS: 513 patients over a total of 182 patient years, 3328 general practitioner (GP) contacts, 3655 reasons for encounter. MAIN RESULTS: Prisoners consulted the GP 17 times a year on average (95%CI 15 to 19.4). It is 3.8 times more than a demographically equivalent population in the community. The most common reasons for encounter were administrative procedures (22%) followed by psychological (13.1%), respiratory (12.9%), digestive (12.5%), musculoskeletal (12%), and skin problems (7.7%). Psychological reasons for consultations (n = 481) involved mainly (71%) feeling anxious, sleep disturbance, and prescription of psychoactive drugs. Many other visits concerned common problems that in other circumstances would not require any physician intervention. CONCLUSION: The most probable explanations for the substantial use of primary care in prison are the health status (many similarities noted between health problems at the admission and reasons for consultations during the prison term: mental health problems and health problems related to drug misuse), lack of access to informal health services (many contacts for common problems), prison rules (many consultations for administrative procedures), and mental health problems related to the difficulties of life in prison.