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This is a retrospective descriptive - analytical study on determination of the relationship between breast cancer and first class familial background in women suffered from breast cancer in hospitals of Tehran university of medical sciences in 1997.The investigated units consisted of 659 women suffered from breast cancer. The samples were randomly assigned into 2 groups, 320 women suffering from breast cancer in case group and 339 healthy women ranging in age from 25 to 64 in control group. The following criteria in two groups were considered: age, pregnancy before 35 years of age, the nonexistence of late menopause and pubertas praecox, and not using contraceptive pills.The background of the first class familial of the samples were compared. Furthermore, the background of breast cancer in mother, sister, mother and sister, two sisters (unilaterally or bilaterally) before and after menopause was compared.To collect data, a questionare was used.The results indicated that there was a significant statistically relationship between breast cancer and first class familial background (P=0.0002). Besides, the background of breast cancer in mother, sister, mother and sister unilaterally and before menopause was significantly related to suffering the patients from breast cancer (PO.05). However, there was no significant relationship between the background of breast cancer in two sisters unilaterally before and after menopause and suffering the patients from breast cancer.