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The familial risk of female breast cancer is somewhat less than 2.0 when a first-degree relative is diagnosed with breast cancer, but it is not known to what extent heritable or environmental factors explain the familial clustering. Such data would be valuable for prevention and gene identification strategies.We used the nation-wide Swedish Family-Cancer Database on 10.2 million individuals and 190,000 mothers' and 26,000 daughters' breast cancers to calculate familial standardised incidence ratios (SIRs), for all invasive breast cancers in daughters, who were 0-66 years old. Over 5500 familial breast cancers were recorded.The familial SIR for all invasive breast cancer was 1.71 by breast cancer in the mother only, 1.95 by breast cancer in a sister only, and 2.75 by breast cancer in both a mother and sister. The SIRs did not change when adjustments were done for period, age at first birth, parity, socio-economic status and region. Age difference between sisters showed a small variation in risk for breast cancer but the highest SIR was found for those whose age difference was from 6 to 10 years. Half sisters showed an excess of familial risks exactly half of full sisters, the SIR being 1.44.These data suggest that familial aggregation of breast cancer is mainly due to heritable causes. Because the known susceptibility genes only explain about a quarter of the familial aggregation, the remaining majority offers a challenge to new genomic approaches.