OBJECTIVES: To assess the long-term safety of discontinuation of secondary anti-Pneumocystis prophylaxis in HIV-infected adults treated with antiretroviral combination therapy and who have a sustained increase in CD4 cell counts. DESIGN: Prospective observational multicentre study. PATIENTS AND METHODS: The incidence of P. jirovecii pneumonia after discontinuation of secondary prophylaxis was studied in 78 HIV-infected patients on antiretroviral combination therapy after they experienced a sustained increase in CD4 cell counts to at least 200 x 10(6) cells/l and 14% of total lymphocytes measured twice at least 12 weeks apart. RESULTS: Secondary prophylaxis was discontinued at a median CD4 cell count of 380 x 10(6) cells/l. The median follow-up period after discontinuation of secondary prophylaxis was 40.2 months, yielding a total of 235 person-years of follow-up. No cases of recurrent P. jirovecii pneumonia occurred during this period. The incidence was thus 0 per 100 person-years with a 95% upper of confidence limit of 1.3 cases per 100 patient-years. CONCLUSIONS: Discontinuation of secondary prophylaxis against P. jirovecii pneumonia is safe even in the long term in patients who have a sustained immunologic response on antiretroviral combination therapy.