Adenoid cystic carcinoma (ACC) of the breast is a rare variant of breast malignancy and is known to have an excellent prognosis. We report two cases of ACC diagnosed by preoperative fine-needle aspiration cytology (FNAC), which proved to be very useful in determining the appropriate treatment. The patients were a 57-year-old woman (case 1) and a 71-year-old woman (case 2). On physical examinations and imaging studies both tumors were recognized as lobulated tumors that measured 3.0 x 2.3 cm (case 1) and 3.9 x 3.4 cm (case 2) respectively. FNAC materials showed clusters of malignant cells surrounding globules of mucus, therefore, ACC was diagnosed. Considering the characteristics of ACC, breast-conserving surgeries with axillary dissection and adjuvant radiotherapy were performed instead of primary chemotherapy or mastectomy. Histologically, a distinctive biphasic pattern was observed that consisted of true laminae and pseudocystic spaces. Tumor sizes were 4.0 x 3.3 cm (case 1) and 4.6 x 3.8 cm (case 2), respectively, and surgical margins were negative on microscopic examination. Lymph node metastasis was not present in either case. Even though ACC is very rare, preoperative diagnosis can be made based on its characteristic features. Preoperative diagnosis is extremely useful for determining appropriate treatment.