Pneumocystis pneumonia occurred in five adults. Each of them was receiving immunosuppressive drug therapy for underlying lymphoma or systemic lupus erythematosus at the time pneumonitis became evident. Premortem diagnosis of infection was made in three of the patients by open-lung biopsy, and two were treated with specific therapy. Two other patients hospitalized in the same room showed signs of Pneumocystis infection within one week of each other. This experience and data from studies of pneumocystosis within families and in European nurseries suggest that contagion may play a role in acquisition of the infection, although experimental animal models and other reported cases in man imply that activation of already latent infection with Pneumocystis accounts for overt disease. A review of the literature acquaints the reader with problems in diagnosis, treatment, and epidemiology of the disease.