Fine needle aspiration(FNA) is an effective tool in diagnosing mammary carcinoma. We experienced 7 cases of histologically confirmed mammary mucinous carcinoma among 3,052 aspirated cases of breast from 1992 to 1996 in Asan Medical Center. The average age of the patient was 48(33-64) years. The mean size of the lesions was $1.6(0.7{sim}3)cm$, and they were palpated as well-defined, firm to hard masses. The cytologic features that may be useful in making a FNA diagnosis of mucinous carcinoma of the breast were analysed. Mucinous background and tumor cell clusters with occasional single cells were observed in all cases. Among them, two cases showed abundant scattered single cells, whereas only few single cells were seen in the other two cases. Tumor cells exhibited mild pleomorphism in four cases and moderate pleomorphism in three cases. Nucleoli tended to be not prominent and are observed in three cases, rarely noted in other three cases and not seen in one. There was microcalcification in four cases(57%). In conclusion, mucinous background and clustered tumor cells showing mild to moderate pleomorphism with characteristic clinical findings allow us to diagnose mucinous carcinoma of the breast.