The clinical and pathologic features of 31 breast lesions composed of a prominent proliferation of myoepithelial cells either admixed with epithelial cells or in pure form were studied. The lesions were divided into three categories: myoepitheliosis, adenomyoepithelioma, and malignant myoepithelioma (myoepithelial carcinoma); the latter is the only lesion composed purely of myoepithelial cells. Three multifocal, microscopic lesions located in the peripheral duct system were designated as myoepitheliosis. Twenty-seven solitary, grossly palpable, predominantly centrally located lesions qualified as adenomyoepithelioma. These were further subdivided into spindle-cell, tubular, and lobulated variants. Two lesions in the latter group had a carcinoma arising within them. Only one case, which was characterized by a solitary mass composed of an infiltrative spindle cell proliferation, qualified as malignant myoepithelioma (myoepithelial carcinoma). Two patients with adenomyoepithelioma developed recurrences; one tumor was of the tubular type, the other of the lobulated type. Both of these tumors had irregular margins. One of these patients had two recurrences and is currently well 8.5 years after the initial excision. The second patient developed a recurrence 8 months after initial excision; the recurrence presented as multiple nodules. One of the patients with myoepithelial carcinoma arising in an adenomyoepithelioma also developed a recurrence within 2.3 years. Her initial tumor was located in the axillary tail of the breast, and she had axillary node metastasis at the time of presentation. All remaining patients with follow-up are well without evidence of recurrence up to 17.3 years after the initial diagnosis (average follow-up, 6.1 years); one patient died of unrelated causes.