Twelve cases of pure adenoid cystic carcinoma of the breast were reviewed. Patients ranged in age from 34 to 69 years. Seven carcinomas were in the right breast, and five in the left; five of the 12 were located in the central region of the breast, five in the upper outer quadrant, and the two in the upper inner and lower inner quadrants, respectively. Average diameter of the primary tumors was 2.5 cm (range, 0.7 to 6.0). We graded the tumors according to a system used for adenoid cystic carcinoma of the salivary gland: five tumors were grade I, six were grade II, and one was grade III. An average of 5 years after diagnosis, all patients with grade I tumors were either alive without evidence of disease or had died of unrelated causes. Among the six patients with grade II tumors, one developed a local recurrence 5 years after diagnosis and subsequent pulmonary metastasis, and one died of metastatic adenoid cystic carcinoma 13 years after diagnosis. The one patient with grade III tumor had shown metastases in axillary lymph nodes at mastectomy, and she died of disease 2 years later. These findings suggest that the grading of adenoid cystic carcinoma of the breast may be important in prognosis and treatment selection.