PURPOSE: High false negative results on fine needle aspiration cytology (FNAC) have been reported in the diagnosis of breast cancers, which are small sized, desmoplastic type, infiltrating lobular cancers, tubular cancers, and mucinous cancer. METHODS: From Jan. 1997 to Dec. 1998, we performed 622 FNACs at the Department of Surgery, Korea University Hospital. Among those aspirates, 246 cases were followed by subsequent pathological confirmation and were included in this study. Pathological reports for breast specimens were reviewed for the size, the location, the grade or the pathological subtype, and the presence of fibrosis in the breast lesions. The pathological characteristics were compared statistically with the results of the fine needle aspiration cytology. RESULTS: The likelihood ratios for malignant, suspicious, atypical, benign, and unsatisfactory cytological diagnoses were 98.7, 5.5, 1.1, 0.07, and 0.6, respectively. The absolute and the complete sensitivities for the malignant lesions were 64.5% and 90.3%, respectively. The rate of unsatisfactory diagnosis was 9.3%, and the false-negative rate was 4.3%. The concordance rates of FNAC results were 75% for tumors less than 1 cm in size, 60.7% for 1-2 cm, 76.5% for 2-5 cm, and 50% for greater than 5 cm (p=0.01). The location of the tumor also affected the FNAC result, and the highest concordant rate was found in tumors of the lower outer quadrant of the breast (73.3%, p=0.001). The Bloom-Richardson grade and histologic type of the breast cancer also affected the FNAC result. Low-grade tumors, medullary carcinomas, metaplastic carcinomas, lobular carcinomas, adenoid cystic carcinomas, and mucinous carcinomas usually showed discordant FNAC results. The presence or the absence of fibrotic pathology did not affect the FNAC result (p=0.39). CONCLUSION: The sensitivity of FNAC was 90.3% in the diagnosis of breast cancer and the false negative rate was 4.3%. The greatest concordance of FNAC diagnosis was found in the patients with tumor less than 5 cm in size and located in the axilla and locoregional recurrent area. The FNAC results for breast cancer were frequently incorrect for low-grade tumors and cancers of a rare pathological type.