Women who have relatives known to have had breast cancer are at an increased risk of getting the disease compared with the general population. On the basis of an extensive collection of family trees of women with breast cancer, the magnitude of this increase in risk is computed. Previously published results on other breast cancer risk factors are drawn upon and it has been possible to take account of some of these e.g., age, decade of birth, age at first childbirth and parity in the risk estimates. The relative risk with such adjustment is found to be 2.59 for mothers and 2.56 for sisters.