Five renal transplant recipients developed Pneumocystis carinii pneumonia (PCP) over a 22-month period, while no cases had been observed over a 5-year period in 114 transplanted patients treated with the same immunosuppressive protocol. All patients were HIV-negative, and no modification in diagnostic techniques for P. carinii could account for this observation. All five patients developed PCP within 2 months of an acute graft rejection episode. All of them attended the same outpatient facility as AIDS patients attending the hospital, where they shared the waiting and treatment rooms. Comparison of cases with matched controls was possible in three instances and revealed that the cases had had more outpatient clinic encounters with AIDS patients who had presented, or subsequently developed, PCP. This observation suggests that AIDS patients developing PCP may transmit the infection to other immunosuppressed patients.