Tumor response is one of the most important criteria in the analysis of chemotherapy. A chest computed tomographic (CT) scan and fiberoptic bronchoscopy (FOB) might give different results, as they analyze different aspects of the effects of chemotherapy on lung cancer. The response of the chest tumor in 103 patients with stage III or IV lung cancer (35 with small-cell lung cancer [SCLC] and 68 with non-small-cell lung cancer [NSCLC]) who prospectively entered chemotherapy trials was studied in order to determine the concordance between the chest CT scan and FOB. The chest CT scan allowed an assessment of tumor response in almost all patients, whereas FOB was not able to evaluate this response in 15 of the 103. The frequency of an evaluable endobronchial lesion did not depend on histology (SCLC, 97 percent; NSCLC, 93 percent; chi 2 = 0.85; not significant [NS]) or tumor T classification (T1-2, 83 percent; T3, 94 percent; T4, 97 percent; chi 2 = 1.49; NS). Tumor location in the bronchial airway did not differ when SCLC and NSCLC were compared. Thus, it is not possible to predict a subgroup of patients in whom FOB may be optional. In the group of 88 patients who were evaluable for response using both FOB and CT scan, a statistical concordance of the response classification was observed. The response was overevaluated by CT scan in 22 patients for whom data obtained by FOB appeared to be critical in the evaluation of tumor response. The concordance of response data obtained when the 2 methods were used was lower in NSCLC in comparison with SCLC. Thus, the use of FOB in the analysis of tumor response might be important, especially for NSCLC, inasmuch as FOB modulates the CT-evaluated response.