Despite a consensus on the need for Pneumocystis carinii pneumonia (PCP) prophylaxis for HIV-infected patients with fewer than 200 CD4+ lymphocytes/mm3, the number of cases of PCP has remained stable, and about 70% involve patients who are not receiving prophylaxis. The aim of this study was to determine why these patients are not covered by prophylaxis. It was based on a retrospective analysis of semidirective interviews with 32 patients who developed PCP while not receiving prophylaxis. The reasons given were a lack of knowledge of risk factors for HIV infection, the fear of HIV testing, unawareness of the existence of PCP prophylaxis, a refusal to see a doctor in the absence of symptoms, unwillingness to be monitored and in four cases a failure of the doctor to prescribe prophylaxis.