The aims of this study were to estimate the familial relative risk of breast cancer according to the age of the at risk individual and the age at which the relative was affected, and to estimate the proportion of the general population in several breast cancer risk categories because of a family history and, thus estimate the potential to reduce the overall breast cancer burden using interventions targeted at women at increased risk because of family history. Familial relative risks were computed by comparing breast cancer incidence in relatives of 2809 breast cancer cases from a population based case series with that expected from general population incidence rates. The proportion of the general population in different categories of risk according to family history was estimated from the relative risk of breast cancer for that category and the proportion of cases in that category. 389 (13.8%) cases had at least 1 first degree relative with breast cancer. The relative risk of breast cancer in sisters of index cases was 1.90 (95% confidence interval (CI) 1.63-2.36) and that in mothers 1.73 (1.52-1. 97). The risk to mothers of cases diagnosed under 50 years of age tended to decrease in older mothers, but no age effect was seen for mothers of cases diagnosed >/=50 years of age. There was no evidence that relative risk to sisters declined with age. For women with 2 affected first degree relatives the relative risk was 2.85 (2.12-3. 76). From these data, we estimate that in the general population 6. 8% of women under the age of 50 years and 9.7% of women aged 50-65 years have at least 1 first degree relative affected with breast cancer. Two per cent of women under 50 years have a family history which confers an increased risk of at least 2.5-fold. An intervention targeted at this group that reduced breast cancer morbidity by 20% would reduce the total burden of breast cancer in this age group by 1.1% at most. A family history of breast cancer is quite common in the general population, but preventive interventions targeted at women at high risk of breast cancer because of family history will have limited impact on breast cancer morbidity as a whole.