A light and electron microscopic study of multiple tumorlets in a 66 year old female is presented. Clinically the patient had had chronic obstructive lung disease with bronchiectasis of the right upper lobe. Aside from severe bronchiectatic alterations, the resected lobe showed typical tumorlets. On electron microscopic study, these were shown to consist of cells containing large numbers of a neurosecretory type of granule, which was identical to those present in the bronchial counterpart of the intestinal argentaffin (Kulchitsky) cell, as well as in pulmonary carcinoid and oat cell carcinoma cells. Our studies suggest the possibility of a histogenetic relationship between tumorlets, bronchial carcinoid tumors, and oat cell carcinoma, the cell of origin in each case being the bronchial counterpart of the intestinal argentaffin cell.