The role of BRCA1 in sporadic breast and ovarian cancers remains elusive. Direct involvement of BRCA1 in the development of breast and ovarian cancer is suggested by the finding that the BRCA1 promoter region CpG island is methylated in a proportion of breast and ovarian cancers. The aim of this study was to compare the incidence of BRCA1 promoter region methylation in tumours in which loss of BRCA1 has been shown to play a role in pathogenesis (breast and ovarian carcinomas) with the incidence in tumours in which BRCA1 is unlikely to play a role in pathogenesis. Promoter region hypermethylation was significantly more common (P < 0.008) in breast and ovarian cancer (6/38 tumours methylated) than in colon cancer (0/35 tumours methylated) or in leukaemias (0/19 samples methylated). The restriction of BRCA1 promoter region hypermethylation to breast and ovarian cancer is consistent with a pathogenetic role of BRCA1 promoter methylation in these tumours. We suggest that the rarity of observed BRCA1 mutations in sporadic breast and ovarian cancer is due to the greater likelihood of BRCA1 inactivation by non-mutational mechanisms such as methylation.