The possible transmission of Pneumocystis carinii f. sp. hominis from patients with P. carinii pneumonia to asymptomatic health care workers (HCW), with or without occupational exposure to human immunodeficiency virus (HIV)-infected patients with P. carinii pneumonia, was examined. HCW in a specialist inpatient HIV-AIDS facility and a control group in the general medical-respiratory service in the same hospital provided induced sputum and/or nasal rinse samples, which were analyzed for the presence of P. carinii f. sp. hominis DNA by using DNA amplification (at the gene encoding the mitochondrial large subunit rRNA [mt LSU rRNA]). P. carinii f. sp. hominis DNA was detected in some HCW samples; those with the closest occupational contact were more likely to have detectable P. carinii DNA. P. carinii DNA was detected in one HCW who carried out bronchoscopy over a 2-year period. P. carinii-positive samples were genotyped by using DNA sequence variations at the internal transcribed spacer (ITS) regions of the nuclear rRNA operon, along with bronchoalveolar lavage samples from patients with P. carinii pneumonia hospitalized at the same time. Genotyping identified 31 different P. carinii f. sp. hominis ITS genotypes, 26 of which were found in the patient samples. Five of the eight ITS genotypes detected in HCW samples were not observed in the patient samples. The results suggested that HCW in close occupational contact with patients who had P. carinii pneumonia may have become colonized with P. carinii. Carriage was asymptomatic and did not result in the development of clinical disease.The fungal pathogen Pneumocystis carinii f. sp. hominis is primarily associated with pneumonia in the profoundly immunocompromised, e.g., those with human immunodeficiency virus (HIV) infection, and also in patients undergoing organ transplantation or chemotherapy for malignant disease. Recent data suggest that exposure to P. carinii f. sp. hominis is frequent and that reinfection with different isolates of P. carinii f. sp. hominis commonly occurs (14,15,17,35). Little is known about the life cycle of the fungus; the reservoir of infectious P. carinii f. sp. hominis has not yet been elucidated nor have the modes of transmission of the infection.It is now widely accepted that the P. carinii organisms that infect each mammalian species are host specific and that the infection in humans is not acquired from an animal reservoir (32, 39). Airborne acquisition of P. carinii infection has been demonstrated by using the rat model (13). The existence of airborne P. carinii organisms has been supported by the identification of P. carinii DNA in samples of airborne spores from rural environments (38), from animal facilities housing immunosuppressed rats with P. carinii pneumonia, and in hospital inpatient and outpatient rooms used for treating patients with P. carinii pneumonia (1, 2, 16, 29).P. carinii has been found in low numbers in the lungs of patients who did not have pneumonia, both HIV-infected individuals and patients who were only mildl...