16 episodes of pneumonia of obscure origin in 15 immunosuppressed renal patients were investigated without complication by fibreoptic bronchoscopy with alveolar lavage under local anaesthesia. Cytological examination and culture of lavage fluid produced a rapid definitive diagnosis in 15 episodes of pneumonia caused by infections with Pneumocystis, fungi, tuberculosis and staphylococci, or by pulmonary haemorrhage. This led to successful treatment of 10 of the 13 infective episodes.