Fiberoptic bronchoscopy was performed before and during administration of chemotherapy in 32 patients with unresectable non-small cell carconoma of the lung. Pretreatment findings varied with the histologic cell type. Direct visual and/or pathologic evidence of cancer was obtained in 11 of 11 patients with epidermoid, in 5 of 7 with large cell, and in 9 of 14 with adenocarcinoma. In 5 of the 32 patients, intrathoracic tumor was documented at bronchoscopy but not by chest x-ray. During chemotherapy, one of five episodes of response and eight of 21 episodes of chest tumor progression were detected solely by bronchoscopy, while in an additional two objective responses and six progressions, bronchoscopic and radiographic findings simultaneously improved or deteriorated. The likelihood of documenting disease progression by bronchoscopy also depended upon the histologic type of cancer. Enlarging chest tumor found solely by chest x-ray occurred exclusively in patients with large cell carcinoma and adenocarcinoma. During chemotherapeutic treatment of our pateints, addition of serial bronchoscopic examinations to standard means of assessing tumor response frequently allowed the earlier discontinuation of an ineffective drug regimen.