Among total 108 cases of biopsy-proven fibroadenomas of the breast, which obtained from the files of the Asan Medical Center during one year period from October 1998 to September 1999, 23 cases cytologically diagnosed as high risk group were reviewed to retrieve the mis-leading factors. Initial cytologic diagnoses of 23 cases were proliferative breast lesion with atypia(high risk) in 21 cases(91.3%) and papillary neoplasm in 2 cases(8.7%). When we reanalysed 23 cases by Masood scoring system, they were classified as one non-proliferative breast lesion(4.3%), 16 proliferative breast lesions without atypia (69.6%), and 6 proliferative breast lesions with atypia(26.1%). None were subject to the category of carcinoma. Cytologic features leading to the overdiagnosis of high grade epithelial lesions were as follows; cellular dissociation without nuclear atypia, nuclear pleomorphism, anisonucleosis, and occasional macronucleoli without nuclear enlargement, lack of myxoid stroma, and few naked stromal cells. To avoid cytologic overdiagnosis of fibroadenoma, mild to moderate nuclear pleomorphism without nuclear enlargement, and cellular dissociation without nuclear atypia should not be regarded as criteria of high risk group.