Recently, the breast cancer susceptibility gene BRCA2 has been identified in chromosome 13q, a region that also contains the retinoblastoma gene RB1. To elucidate a possible role of BRCA2 and RB1 in sporadic breast tumorigenesis, allelic imbalance (AI) at 13q loci was examined in 78 primary sporadic breast tumors. AI was found in 52-63% of tumors. Nine tumors showed AI only in the BRCA2 region but not at RB1. Six tumors showed AI at RB1 but not in the BRCA2 region. AI in the BRCA2 region correlated significantly with aneuploidy (P = 0.032) and AI at RB1 with small tumor size (P = 0.025). Our data suggest that BRCA2 and RB1 may be both distinct target loci for AI on chromosome 13 in sporadic breast cancer.