The diagnostic accuracy of fine needle aspiration cytology (FNAC) had been reported about 88-99% in the preoperative diagnosis of breast cancer. High false negative results were reported in the diagnosis of breast cancers, which were small size desmoplastic type, infiltrating lobular cancer, tubular cancer and mucinous cancer. The purpose of this study is to define the possible histopathologic characteristics of breast cancer affecting the results of fine needle aspiration cytology. From Jan. 1997 to Dec. 1998, we performed 622 cases of FNAC at the department of surgery, Korea University Hospital. Among these aspirate, 246 cases were followed by subsequent pathological confirmation and were included in this study. The results of fine needle aspiration cytololgy were classified as benign, suspicious and malignant cells. Pathological reports for breast specimens were reviewed for the size, the location, grade or pathological subtypes and presence or fibrosis in the breast lesions. The pathological characteristics were compared statistically with the results of fine needle aspiration cytology. The likelihood ratios for malignant, suspicious, atypical, benign and unsatisfactory cytological diagnosis were 98.7, 5.5, 1.1 and 0.6, respectively. Absolute and complete sensitivities for the malignant lesions were 64.5% and 90.3%. The specificity was 71.9%. False negative and positive rates were 4.3% and 0.7%. The predictive value for malignant cytology in malignancy was 98.4%. The rate of unsatisfactory diagnosis was 9.3%. The range of tumor size is from 0.6 cm to 6.5 cm. The concordance rates of FNAC results wee 75% of less than 1 cm, 60.7% of 1-2 cm, 76.5% of 2-5 cm, and 50% of greater than 5cm of tumor, respectively (p=0.01). The location of tumor also affect the FNAC result and the highest concordant rate was found in the tumor of lower outer quadrant of breast (73.3%, p=0.001). The Bloom-Richardson grade histologic type of breast cancer also affect the FNAC result. Low grade tumor, medullary carcinoma, metaplastic, carcinoma, lobular carcinoma, adenoid cystic carcinoma and mucinous carcinoma usually showed discordant FNAC results. The presence or absence of fibrotic pathology did not affect the FNAC result (p=0.39) In conclusion, the sensitivity of FNAC was 90.3% in the diagnosis of breast cancer and the false negative rate was 4.3%. The great concordance of FNAC diagnosis was found in the patients with tumor of less than 5cm and located in the axilla and locoregional recurrent area. The FNAC results of breast cancer were frequently incorrect in the low grade tumor and cancer of rare pathological type such as lobular, medullary, mucinous and adenoid cystic carcinoma of breast.