Pneumonopathic conditions in the immunocompromised host (IH) are frequent and often serious. Rapid diagnosis is essential and is made possible by bronchoalveolar lavage (BAL). Sixty-two pneumonopathic episodes in 53 immunocompromised patients were examined by BAL, for viral cytopathogenic effects (CPE) in isolated cells, with appropriate viral culture techniques. Viral culture was positive in seventeen of the eighteen episodes in renal allograft recipients and AIDS patients as against eight of the fourty-four episodes in other causes of IH (p less than 0.001). CPE was found thirteen times; in seven cases it was characteristic of cytomegalovirus. Positive viral culture and CPE were shown simultaneously during thirteen episodes in eleven patients. Ten patients died (autopsies performed in three cases confirmed viral presence). Positive viral culture with absence of CPE was observed in twelve cases. There were only four fatalities in this group (the autopsies performed in three cases did not establish the presence of a virus in the pulmonary parenchyma). The percentage of lymphocytes was high in both groups of patients (18.6 +/- 2.8%). CPE is a simple and rapid examination for the diagnosis of viral pneumonopathology in the IH. Prognosis at present is gloomy; more complex examinations such as viral cultures and/or identification of the virus by immunofluorescence will be indicated only when effective antiviral agents become available.