The incidence of the location within the bronchi related to the cell types was investigated with the flexible fiberoptic bronchoscope in 355 cases of lung carcinoma. In 5 patients carcinoma was situated only in the trachea. In the other 350 cases the cell types other than adenocarcinoma were found to show different locations following their cell type. Epidermoid carcinoma was found more frequently in the two upper lobes (p less than 0.001), while small cell carcinomas showed predilection for the main bronchus on the right side, and the upper lobe in the left (p less than 0.001). No difference could be found between the upper, lower lobes and main bronchi for adenocarcinoma. It was also observed that large cell carcinomas were situated more often in the right upper lobe. The most important finding in this investigation was that, apart from adenocarcinoma, the other types were located mainly in the upper lobes, and much less frequently in the lower lobes. The predilection of localization of epidermoid and small cell carcinomas in the upper lobes suggests a possible relationship to tobacco smoke inhalation as these regions have been shown to be more affected by the smoke.