Micronuclei [MN] in lymphocytes of patients suffering from fibroadenoma [n=29] and breast cancer [n=37] was compared with the data from healthy individuals [n=10]. Healthy controls lymphocytes were exposed at G0 stage to different doses [0.5 Gy, 1.0 Gy, 2.0 Gy and 4.0 Gy] of γ-rays. Lymphocytes were cultured for 72 hours to obtain MN with cytochalasin B. A dose dependent increase in the incidence of MN frequency and a decrease in the survival of lymphocytes. Frequency of PBLs [Peripheral blood lymphocytes] with one, two, three and four MN is analyzed. In comparison to baseline, the radiation exposed samples showed higher frequency of MN in control, fibroadenoma and breast cancer. The degree of increase was 4.7 times in control, 10.8 in fibroadenoma and 15.2 in breast cancer as compared to samples without radiation. The overall degree of increase in MN frequency from control to breast cancer was 3.2 times. Non irradiated and irradiated samples showed an increase in MN frequency from stage I to stage IV. Influence of age on MN frequency was not significant. MN assay is an additional diagnostic, prognostic tool in benign and malignant tumors of breast to assess the degree of genetic damage and radiosensitivity.