In a 12-month period, brush biopsy through a fibreoptic bronchoscope was performed on 125 consecutive patients who were clinically or radiographically suspected of lung cancer. Of the patients, 62 appeared to have lung cancer. Cytological analysis of the brush-biopsy specimens was positive in 69%. A total of 58% had positive biopsy, and 58% had cancer cells in the bronchial secretion aspirated during the bronchoscopy. Thirty-five per cent had positive mediastinoscopy. When the methods of examination were combined, the diagnosis could be established in 90% of the patients. The study shows that brush biopsy and cytological analysis, in combination with the other methods of examination, increase the diagnostic sensitivity in bronchoscopically visible tumours. In bronchoscopically invisible tumours, brush cytological analysis, together with the study of bronchial secretion and fine-needle puncture, is the most effective diagnostic method. The diagnostic possibilities can presumably be further improved by the use of fluoroscopic control in two places during the bronchoscopy.