Greenberg, S. Donald, Smith, Marilyn N., and Spjut, Harlan J.: Bronchiolo-alveolar carcinoma—cell of origin. Am J Clin Pathol 63: 153–167, 1975. Bronchiolo-alveolar carcinoma is the least common of the primary pulmonary carcinomas, and there is controversy as to its cell of origin. In this light- and electron microscopic study of five bronchiolo-alveolar carcinomas, at least two cell types were found, both of bronchiolar origin. One cell type is a metaplastic bronchiolar mucous cell and the other a bronchiolar stem cell that has ultrastructural features of both the respiratory ciliated and the respiratory nonciliated (“Clara”) cell. It would not be unusual if tumors of the bronchiolo-alveolar region differentiate into cells of either the bronchiole or the alveolus, for embryologically they have a common origin. However, as information about the ultrastructure of these tumors accumulates, it is becoming apparent that an alveolar-cell carcinoma must be a rare occurrence. Hyperplastic Type II alveolar epithelial cells may be found about the margins of these tumors and can be mistaken for the neoplastic cells.