Serial fiberoptic bronchoscopic examinations were performed during intensive chemotherapy with a combination of drugs in 77 previously untreated patients with small cell carcinoma of the lung. Before treatment, bronchoscopic examination revealed evidence of cancer in 93 percent (70) of the 75 patients studied at that time, including 8 percent (six) in whom the tumor was not evaluable on the chest x-ray film. After therapy was initiated, 36 percent (29) of the 81 procedures performed in patients with a complete response radiographically and 62 percent (33) of the 53 bronchoscopic procedures in those with a partial response or no response showed evidence of tumor. In both of these groups, patients with abnormal findings on endoscopic examination had a much higher rate of relapsing tumor of the chest within a 12-week period. Progression of intrathoracic tumor was first detected solely by bronchoscopic examination in 22 percent (seven) of the 32 episodes of progression. In our hands, repeated fiberoptic bronchoscopic procedures during chemotherapy for small cell carcinoma have yielded information not apparent from the chest x-ray film in a significant number of patients.