Ultrastructural examination of five adenoid cystic carcinomas, three breast and two salivary gland, reveals identical patterns of tumour cell differentiation, organization and distribution of cellular products (Zaloudek, Oertel & Orenstein 1984). In both sites, there is proliferation of two populations of cells, one with characteristics and organization of duct-type luminal epithelial cells and a second that forms the principal proliferating component and has the overall organization and appearance that would suggest that they represent modified myoepithelial cells. Recent ultrastructural studies also indicate that tumour cell types and histological organization similar to those described for adenoid cystic carcinoma occur during histodifferentiation of salivary gland pleomorphic adenoma (Dardick et al. 1983a, b). The characteristic histological pattern of adenoid cystic carcinoma is dependent on the formation of pseudolumina containing proteoglycans and reduplicated basal lamina. Similar, but smaller, lumina of like organization and contents are evident in some cases of pleomorphic adenoma. Both the ultrastructural similarities of the tumour cell types and their organization, in adenoid cystic carcinoma and pleomorphic adenoma, suggest that these tumours have a similar histogenetic basis. The fact that one lesion is malignant and the other benign does not preclude common types of tumour cells and developmental processes.