This study aimed to determine the reasons for sampling and interpretative errors in false negative and false positive diagnoses of breast carcinoma on fine-needle aspiration cytology (FNAC) material. The study design is that a totally 912 cases of breast FNAC were performed between 2000 and 2004, and 126 cases of them were diagnosed as breast carcinoma. Only those cases with cytohistological discrepancies were cytologically reviewed, in which the cytological material was abnormal and to some extent misinterpreted or both. There were 8 false negative diagnoses (false negative rate 6.3%) and 3 false positive diagnoses (false positive rate 2.3%). The results of this study showed that among 8 false negative cases, 5 showed hypocellular smears with minimal nuclear pleomorphism of the cells. Histology revealed 3 infiltrating ductal carcinomas of scirrhous subtype and 2 infiltrating lobular carcinomas. The smears of other 2 false negative cases, which histologically verified as well-differentiated infiltrating ductal and pure intraductal carcinomas, were hypercellular and composed predominantly of groups of cohesive, small, and uniform cells simulating fibroadenoma or fibrocystic changes. Smear of the last false negative case (histologically verified as infiltrating ductal carcinoma with extensive cystic degeneration) revealed large sheets of macrophages and degenerated epithelial cells on inflammatory background. In 3 false positive cases, 2 were histologically proved as fibroadenoma and 1 fibrocystic changes. Smears of the 2 false positive fibroadenomas showed very high cellularity, overlapped clusters, and frequent stripped bipolar nuclei. The fibrocystic case showed tight clusters of apocrine cells and sheets of loosely aggregated macrophages that were over interpreted. The conclusion of this study is that hypocellularity and relatively nuclear monomorphism are the reasons for failure to diagnose breast carcinoma. Careful attention should be paid to extreme nuclear monomorphism and absence of naked bipolar nuclei. So awareness of smear cellularity and subtle cytological features will aid in the correct preoperative diagnosis of lobular; scirrhous; and intraductal carcinomas, and false negative diagnoses can be minimized. A cytologically atypical or suspicious diagnosis together with positive mammographical and clinical findings should suggest a diagnosis of malignancy. Hypercellular smears with overlapped clusters should be carefully assessed for uniformity of the cells and detailed nuclear features. If the full-blown malignant cytomorphological features are not visible, a diagnosis of suspicious or inconclusive should be made and frozen section Created by Wameed Al-Hashimy intraoperative imprint cytology is recommended before surgery.