Three cases of adenoid cystic carcinoma have been identified in a 10-year review of 2686 cases of breast carcinoma. The criteria necessary for diagnosis have been reviewed with particular reference to cribriform intraduct carcinoma and adenocarcinoma of the breast with small, dark, 'basaloid'-cell pattern. The most important single diagnostic criterion of adenoid cystic carcinoma is a biphasic cellular pattern which may be aded by the demonstration of two types of mucin stromal acid mucopolysaccharide and ductal neutral mucopolysaccharide. This tumour most frequently presents as a painful or tender mass near the areola, and it carries a uniquely favourable prognosis when compared with similar tumours elsewhere in the body. Actomyosin has been demonstrated in all three tumours by an immunofluorescent method, and this supports a predominantly myoepithelial origin.