Thirty pulmonary infiltrates in 26 patients were investigated by bronchoalveolar lavage. Sixteen of the patients were on therapeutic immunosuppression for renal disease or transplant and 10 had leukaemia, lymphoma, or allied conditions. A rapid specific diagnosis was made in 21 (70%) episodes by cytological examination of the fluid and in 28 (93%) by a combination of cytology and microbiology. No complications from haemorrhage or pneumothorax ensued. Pneumonia due to Pneumocystis carinii was the most common diagnosis (27%), but opportunistic infections from cytomegalovirus, candida, aspergillus, zygomycetes, and acid fast bacilli were also identified by cytology. Two episodes were caused by occult pulmonary haemorrhage and five patients had malignant infiltration of the lung from leukaemia, myeloma, Hodgkin's disease, and lymphoplasmacytoid lymphoma. In two of these there was also evidence of infection. In seven cases with non-diagnostic cytology infections due to Staphylococcus aureus, Pseudomonas aeruginosa, pneumococcus, micrococcus, and Aspergillus fumigatus were identified on culture. In two patients (7%) no specific diagnosis was established by lavage: one had serological evidence of legionella infection and the second had P aeruginosa septicaemia. Twelve (75%) of the renal patients and six (60%) of those with leukaemia, lymphoma, and allied conditions recovered.