Ultrastructural features were correlated with a series of special staining reactions in eight cases of bronchiolo-alveolar carcinoma. Ultrastructural, all tumors were similarly composed of large cells with abundant cytoplasm and small nuclei in close contact with each other. Straight membranes or complex interdigitations occurred within adjacent tumor cells, attached to each other by scattered desmosomes. Microvilli or cilia abutted from free surfaces of the cells, and were noted in different stages of evolution. Numerous organelles were seen in the cytoplasm, including prominent mitochondria and single or coalescent secretory vacuoles with granular matrix resembling mucin. Other cytosomes less commonly found were irregular, partially lamellated inclusions and dark, homogeneous structures without limiting membranes. The stroma of the tumors was rich in elastin and collagen. Both the number of secretory vacuoles in the cytoplasm of tumor cells and the amount of connective tissue fibrils in the stroma of the tumors correlated well with the findings in the series of special staining reactions. No definite ultrastructural feature was present to identify the tumors as orginating from Type II alveolar epithelial cells, but the possibility exists that they arose in the bronchiole, from undifferentiated basal cells or mucinous cells per se. Our impression in these eight cases studied is consistent with the view that bronchiolo-alveolar carcinomas are indistinguishable at the ultrastructural level from other bronchogenic adenocarcinomas.