Adenoid cystic carcinoma of the breast is an uncommon carcinoma with a distinctive histology. Prognosis is favorable, although recurrence and distant metastases have been described. We assessed whether histologic features and proliferative activity can identify aggressive neoplasms. We studied 31 cases of adenoid cystic carcinoma (age range of patients, 33 to 74 years). Three histologic grades were defined: grade I: completely glandular; grade II: < 30% solid areas, and grade III: > or = 30% solid pattern. In 19 of 31 cases, immunohistochemical stains for estrogen receptor were available. Twelve of 31 cases were immunohistochemically stained for Ki-67 antigen using MIB1 antibody. Ten of 20 tumors were subareolar. All tumors were grossly circumscribed; however, 12 of 20 (60%) had focal infiltration peripherally. Five of 19 tumors were estrogen receptor positive. There was no statistical correlation between MIB1 score and histologic grade, nuclear grade, infiltration of the adjacent fat or breast parenchyma, or estrogen receptor status. All patients were alive with no evidence of disease after a median follow-up of 7 years. Neither histologic or nuclear grading nor proliferative activity were useful prognosticators. None of the tumors had lymph node metastases. Therefore, axillary lymph node dissection may not be necessary. Because more than half of adenoid cystic carcinomas are infiltrative focally, the most important therapeutic goal is complete tumor removal with uninvolved margins of excision.