Forty eight patients with the acquired immunedeficiency syndrome (AIDS) presented to the Mount Sinai Hospital in New York with persistent cough and dyspnoea or an abnormal chest radiograph, or both. Thirty two (67%) were found to have Pneumocystis carinii pneumonia, either alone or in combination with another pathogen. Of these patients, eight (25%) had a normal chest radiograph. Abnormalities in the single breath carbon monoxide diffusing capacity and alveolar-arterial oxygen gradient [A-a) DO2) suggested infection with Pneumocystis carinii. Fibreoptic bronchoscopy with transbronchial biopsy was 100% sensitive in the diagnosis of pneumocytis pneumonia. Fibreoptic bronchoscopy should be undertaken in patients suspected of having a pulmonary complication of AIDS, even if the chest radiograph is normal.