We use the population-based Family-Cancer Database from Sweden to study familial breast cancer. The size of the population and the nation-wide registration of cancer offer unique possibilities for epidemiological studies of familial cancer, including complete and unbiased identification of cases in the probands and in their relatives, and complete and unbiased identification of the family relationships. Using the Database, we wanted to answer the following questions: (i) proportion of familial breast cancer among all breast cancers; (ii) familial relative risks in breast cancer alone or in combination with another cancer, defined either through the mother or the daughter; (iii) modification of familial risk by age; and (iv) effects of paternal breast cancer alone or in combination with maternal breast cancer. The proportion of familial female breast cancer among all breast cancers before 54 years of age in Sweden was 8.7%. The familial relative risk was about 1.8, but is likely to decrease to about 1.5 in the ageing population. The higher familial relative risks were evident in young women, being 4.0 when both the mothers and their daughters were diagnosed at ages F40 years. Paternal breast cancer, in combination with maternal breast cancer, caused a large (but not statistically significant) risk in the daughters. In mothers and daughters, ovarian but not colon cancer was increased in combination with breast cancer. Int.