PURPOSE: Mucinous carcinoma of the breast is relatively rare among the malignant breast lesions. It has distinctive pathological and behavioral characteristics to separate from other types of breast cancers. Accoding to the WHO definition, mucinous carcinoma of the breast is a carcinoma containing large amounts of extracellular epithelial mucus, sufficient to be visible grossly, and recognizable microscopically surrounding and within tumor cells. Mucinous carcinoma shows the characteristic features clinically and histologically because of the mucus production by the tumor. MATERIALS AND METHODS: We reviewed 29 cases of mucinous carcinoma that had been treated from 1985 to 1996 in the Department of General Surgery, Yonsei University College of Medicine. RESULTS: The results were as follows: The prevalent age group was the fifth decade (37.9%). The 27 cases were female and 2 were male. The most common sign and symptom was a palpable mass (100%). The most patients visited the hospital within 2 months of onset (55.2%). Most frequent tumor size was 2-3 cm in diameter, found in 10 cases (34.5%), and 13.8% of cases was more than 5 cm in diameter. The most frequent site of tumor was the upper outer quadrant in 16 cases (55.2%). The operations performed were as follows: Modified radical mastectomy (Auchincloss or Patey) in 24 cases (82.8%), quadrantectomy with axillary lymph node dissection in 4 cases (13.8%), simple mastectomy with lower axillar dissection in 1 cases (3.4%). Axillary lymph node metastasis was present in 3 cases (10.3%). The most common stage at diagnosis was stage IIa in 13 cases (44.8%). CONCLUSION: This study shows some characteristics of mucinous carcinoma of breast distinict from those previously proposed Further studies are needed to identify clinical parameters which characterize mucinous carcinoma of the breast.