An outbreak of Pneumocystis carinii pneumonia (PCP) in three patients within a 6 week period was reported. Two patients had acute lymphoblastic leukaemia and one had brain-stem glioma. They shared common features of immunosuppression and absence of cotrimoxazole prophylaxis and had been nursed in the same room. The severity of PCP and its response to treatment may be related to the degree of immunosuppression. Because of the morbidity and mortality of PCP, chemoprophylaxis should be given to all at-risk cases. Furthermore, isolation of patients with PCP should be considered in view of increasing evidence of nosocomial transmission.