The ProF-SC commenced in 1992 with women from 11,171 families ascertained through population-based and clinic-based sampling and who cover the full spectrum of familial risk of breast cancer. Families were from the United States, Canada, and Australia. At baseline, this cohort of women over the age of 18 years included 18,853 unaffected and 12,787 affected with breast cancer. Multi-generational cancer family histories were sought from all participants who were administered the same baseline epidemiology questionnaire. Blood samples have been collected from 83% of participants, and another 2% have DNA available through buccal samples of participants and used for extensive genetic characterisation.