In a retrospective study of 377 patients with bronchial carcinoma, sputum had been reported positive or suspicious for malignant cells in 59.7%. The most important factors contributing to a positive result were the histological cell type and the site of the tumour as assessed at bronchoscopy. Significantly more positive results were obtained in epidermoid tumours and those with a visible bronchoscopic abnormality. A higher proportion of positive results also occurred in large tumours, lower lobe tumours, and those associated with collapse and consolidation. The physical condition of the patient and the presence of infection in the sputum were not important factors contributing to a negative result. Although adequate numbers of sputum samples were collected the predominant problem was the large number of unsatisfactory sputum specimens. The preparation of the sputum specimens in the laboratory is important and is also a potential source of error.