: Micronuclei scoring can be used as a bio-marker of genotoxic and chromosomal damage. Fine needle aspiration cytology (FNAC) is applied as the primary tool for diagnosis in breast masses because of its ease and rapidity. Micronucleus (MN) scoring is carried out in benign (fibroadenoma) and malignant (infiltrating ductal carcinoma) breast lesions to evaluate the role of Micronuclei as a biomarker in breast carcinomas.: To study Micronuclei (MN) scores as a biomarker on breast cytology aspirated smears. This was a prospective study done for duration of two years in the Department of Pathology, A.C.S Medical College and Hospital, Chennai. The features of micronuclei in 60 breast aspirate smears were studied and compared in benign conditions, proliferative and malignant conditions.The most common diagnosis was of fibroadenoma seen in 38 (63.3%) cases. Adenosis was seen in 10 (16.6%) cases. Usual ductal hyperplasia in 6.6% cases and Invasive ductal carcinomas in 6 (10%) cases. The Average Micronucleus score/1000 cells and the range of micronucleus score was higher in malignancy as compared to benign conditions. : Micronuclei can be used as a biomarker on fine needle aspiration cytology smears of breast lesions. An increase in micronuclei is usually seen in malignant conditions as compared to benign tumours. Attention to features of micronuclei can give a clue to the presence of malignancy.