Pneumocystis carinii pneumonia (PCP) is described almost exclusively in immunocompromised hosts. This report describes our experience with acute episodes and the follow-up of five patients with PCP who had no known predisposing conditions. The average follow-up period was 3.6 years (range, 2.6 to 4 years). Lymphocyte subpopulations (CD4, CD8, and CD4:CD8), serum immunoglobulins (IgG, IgM, and IgA), and serologic studies for human immunodeficiency virus were carried out on all patients at least twice, both at the beginning and the end of the follow-up. None of the patients presented compatible data with AIDS or any other identifiable risk factors. We conclude that PCP can occur in patients who apparently do not have immunosuppression.