Records of histopathology from the 3734 Jewish women having breast biopsy and/or operations in all Israeli hospitals during the year from July 1979 to June 1980 were reviewed. Approximately 28.5% of these women were diagnosed as having breast cancer and 71.5% as having benign breast disease: 48.5% had benign proliferative mastopathy (BPM), 16.6% had fibroadenoma (FA) without coexistent BPM, and 6.4% had other benign breast conditions. The age-specific incidence rate was 66 in 100,000 for breast cancer and 165.2 in 100,000 for benign breast disease. Native European or American women and native Israeli women had significantly higher age standardized incidence rates of both breast cancer and BPM, but not of FA, as compared to African/Asian-born women (P less than 0.01). Age-related ratios between invasive to precursor breast lesions were similar in all ethnic groups. The data suggest that breast cancer and benign proliferative mastopathy may have a common etiologic component.