In a retrospective cohort study, the mothers and sisters of 740 breast cancer patients aged under 36 at diagnosis have been studied for mortality and cancer incidence. Significantly increased breast cancer mortality was observed below age 60 (30 deaths; SMR = 3.4), but not at older ages (four deaths; SMR = 0.9). The cumulative breast cancer incidence in the relatives was 3.6% by age 50, 7.6% by age 60 and 11.6% by age 70. They also suffered excess mortality below age 60 for cancers of reproductive sites (cervix, ovary and endometrium; 15 deaths; SMR = 2.6) and lung (11 deaths; SMR = 3.2), but not for other sites (12 deaths; SMR = 0.9). This large population-based cohort study provides further confirmation of genetic susceptibility to breast cancer at young ages.