Systemic oxidative stress is thought to contribute to the risk of various cancers including breast cancer. The extent of DNA damage reflects a balance between oxidative stress and DNA repair ability which is associated with breast cancer risk. Aim: This study involves assessment of the levels of the markers of the oxidative DNA damage in women with breast tumors. The main techniques presently used are: comet assay to measure the extent of single strand DNA breaks (comet tail length and comet tail moment), 8-hydroxy-2`-deoxy guanosine (OHdG) levels (which is one form of oxidative DNA damage used to detect DNA modification), and numbers of DNA lesions. Subjects and methods: The study included 80 females; of these, 40 were newly diagnosed untreated breast tumors with an age range of 24-75year. They were admitted to the Department of surgery Al-Khadymia Teaching Hospital during the period of December, 2004 to May, 2006. Moreover, 40 healthy controls (age range 24-50 year) with no family history of any type of cancer were involved in the study. Seven to ten milliliters of blood and tumor specimens were taken from 40 newly diagnosed untreated breast tumors patients (age range of 24-75 years) for comet assay and 8-OHdG level. Same tests were done on blood samples which were withdrawn from 40 healthy controls (age range 24-50 years) with no family history of any type of cancer. Leukocytes from control and patients were placed on slides precoated with agarose, covered by another layer of low melting temperature agarose with cells and subjected to electrophoresis PH13 for one hours then the level of DNA damage expressed as comet tail length and moment was assessed and quantified by computer image analysis. The 8OHdG concentration was determined by ELISA using: mouse monoclonal antibody specific for 8-OHdG. The concentration 8OHdG was converted to "number of lesions/106 DNA bases". Results and Discussion: There were highly significant differences in the mean levels of blood and tissue comet tail length and moment in benign and malignant breast tumor as compared to the control groups with an augmented elevation in the breast cancer as compared to the benign breast tumors. Yet, no statistical significant effect of the patient's age, or body mass index (BMI) was observed on the mean comet tail length. Moreover, the mean tissue comet tail length was lower in the benign papilloma as compared with other benign subtypes (3.5_0.31,4.5_0.24 and 4.8_0.17 respectively) and was significantly higher in the invasive malignant breast tumor as compared to noninvasive carcinoma (17.8 _ 0.28 and 16.7_ 0.39 respectively)(p0.001). The mean comet tail moment in the control and malignant breast tumor groups rises with the increase of the age, while there was no effect of the BMI on the comet tail moment. No statistical difference was recorded in the tissue comet tail length among benign and malignant breast tumor subtypes. Measurement of the modified base (OHdG) due to oxidative DNA damage reveals a highly significant elevation in the mean serum 8OHdG values in both benign and malignant breast tumor (27.1_1.91, 152.6_8.06, respectively)as compared to the mean control values (16.7_ 0.54) (p0.0001). Between groups there was a statistically significant high mean serum 8OHdG values in the malignant breast tumor patients compared to the benign breast tumor (p0.0001). Also there was no influence of age and BMI on the level of the 8OHdG in both control and benign groups. Yet, the serum level of 8OHdG in patients with malignant breast tumor increased (but not significant) with the advance of age. Student t-test reveals no significant elevation in the mean serum 8OHdG values in both invasive and non-invasive carcinoma. The number of DNA lesions per 106 DNA bases showed high increase in benign and malignant tumors (0.2_0.01, and 1.2_0.06,respectively) as compared to the control (0.1_0.004) (p0.0001) group. Between groups t-test showed elevated mean number of DNA lesions in malignant tumors as compared with the benign tumors. There were no effects of (age, BMI) on the number of DNA lesions in both controls and patients. Moreover, number of lesions in invasive and non-invasive lobular carcinoma was higher than the ductal invasive and non-invasive carcinoma. Conclusion: comet assays and modified DNA base (8OHdG) tests are useful, sensitive markers for monitoring the severity of DNA strand break and damage in breast tumor and could be used in the future to identify persons with increased cancer susceptibility.