BACKGROUND: Adenoid cystic carcinoma is a rare type of breast cancer that is generally reported in individual case reports or as series from major referral centers. To characterize early diagnostic criteria for adenoid cystic carcinoma and to determine whether breast-preserving surgery with radiotherapy is as effective as mastectomy for eradicating the disease, we reviewed clinical records of a large series of patients treated for adenoid cystic carcinoma of the breast at a large health maintenance organization (HMO) that includes primary care facilities and referral centers. METHODS: Using the data bank of the Northern California Cancer Registry of the Kaiser Permanente Northern California Region (KPNCR), we retrospectively reviewed medical records of patients treated for adenoid cystic carcinoma of the breast. Follow-up also was done for these patients. RESULTS: Adenoid cystic carcinoma of the breast was diagnosed in 22 of 27,970 patients treated for breast cancer at KPNCR from 1960 through 2000. All 22 patients were female and were available for follow-up. Mean age of patients at diagnosis was 61 years (range, 37 to 94 years). In 17 (77%) of the women, a lump in the breast led to initial suspicion of a tumor; in 4 (23%) of the 22 patients, mammography led to suspicion of a tumor. Median tumor size was 20 mm. Pain was a prominent symptom. Surgical management evolved from radical and modified radical mastectomy to simple mastectomy or lumpectomy during the study period, during which time 1 patient died of previous ordinary ductal carcinoma of the contralateral breast, and 7 died of unrelated disease. At follow-up, 12 of the 13 remaining patients were free of disease; 1 patient died of the disease; and 1 patient remained alive despite late occurrence of lymph node and pulmonary metastases. CONCLUSIONS: Whether breast-preserving surgery with radiotherapy is as effective as mastectomy for treating adenoid cystic carcinoma of the breast has not been determined.