A breast tumour with proven lymph node metastasis is conclusively characterized as an adenoid cystic carcinoma using immunocytochemistry and electron microscopy. The majority of tumour cells showed certain of the characteristic features of myoepithelial cells while the pseudocystic spaces contained large amounts of reduplicated basal lamina. A small proportion of tumour cells, however, showed epithelial differentiation with the formation of true lumina.