To investigate the coordinated occurrence of loss of heterozygosity (LOH) at the BRCA1 locus and microsatellite instability (MI) in sporadic breast carcinomas, 56 tumors were analysed for both genetic alterations. The comparison of clinicopathological features with the obtained data revealed that LOH at the BRCA1 locus was significantly correlated with features specific for familial BRCA1 tumors and with absence of hormone receptors. No correlation was found between LOH and MI. These results suggest that sporadic and familial breast tumors, where BRCA1 is altered, could display similar clinicopathological features and that LOH and MI are distinct genetic events in sporadic breast carcinogenesis.