In an open prospective study in the AIDS Outpatient Department of the Frankfurt University Clinic the efficacy of pentamidine aerosol at a dosage of 200 mg every 14 days in the primary and secondary prophylaxis of pneumocystis carinii pneumonia was tested. The incidence of pneumocystis carinii pneumonia was reduced by more than 75% in the group of patients receiving pentamidine additionally by inhalation as compared with a historical cohort given azidothymidine alone. The results of the study suggest that a CD4-positive cell count of less than 50/microliters with a concomitant secondary lung infection promotes the development of pneumocystis carinii pneumonia. There was no correlation between the incidence of pneumocystis carinii pneumonia and the number of previous disease episodes. No systemic toxicity of pentamidine aerosol was established.