Reliable determination of familial risks for cancer is important for clinical counselling, cancer prevention and understanding cancer aetiology. Family-based gene identification efforts may be targeted if the risks are well characterised and the mode of inheritance is identified. Early-onset breast cancer in a family member is a risk indicator for cancer among first-degree relatives; however, the familial risk pattern has not been assessed fully in population-based incidence studies. We estimated the risks for cancers of the breast, ovary and other sites among the first-degree relatives of 8868 patients in whom breast cancer was diagnosed before they reached the age of 50 years (diagnosed during the period 1943-1999). Population registers and parish records were used to identify 31,235 first-degree relatives, who were followed up to 31 December 2002 for occurrence of cancer by linkage to the Danish Cancer Registry. The observed incidence rates were compared with national rates adjusted for age, sex and calendar period. Overall, 39% of the 674 cases of breast cancer and 43% of the 143 cases of ovarian cancer among relatives were associated with a diagnosis of early-onset breast cancer in a family member. Among relatives under 50 years of age, the proportions were 56% and 58%, respectively, and among relatives 50 years or above the proportions were approximately 30% and 10%. In addition, a slightly but significantly increased risk for cancer of the cervix uteri was observed among relatives, and among those under 50 years of age, we found significantly increased risks for cancers of the colon and gall-bladder. In conclusion, the excess risk for breast cancer in first-degree relatives is large and remains sizable in the subgroup of female relatives aged 50 years or older, and that mutations in BRCA1/2 seem to explain only half of breast cancer cases attributable to family history.