BARD1 was first identified as a BRCA1-interacting protein with tumour-suppressor functions. Some association studies suggested that the BARD1 Cys557Ser variant might be associated with increased risk of breast cancer, but the evidence remains uncertain. We found that the BARD1 Cys557Ser variant was carried by 50 of 1,136 cases (4.4%) and 30 of 623 controls (5.0%) from the population-based Australian Breast Cancer Family Study, 14 of 324 (4.3%) cases from the Kathleen Cuningham Foundation Consortium for Research into Familial Breast Cancer (kConFab), and 30 of 760 controls (4.0%) from the Australian Ovarian Cancer Study. Case-control comparisons showed no evidence that the variant frequency differed by case-control status (P >or= 0.3). Segregation analysis of 14 kConFab variant-carrying families containing 157 genotyped individuals provided no evidence of segregation with disease. We conclude that the BARD1 Cys557Ser variant is not associated with breast cancer risk.