To study the involvement of DNA mismatch-repair genes in sporadic breast cancer, matched normal and tumoral DNA samples of 22 patients were analysed for genetic instability and loss of heterozygosity (LOH) with 42 microsatellites at or linked to hMLH1 (3p21), hMSH2 (2p16), hMSH3 (5q11-q13), hMSH6 (2p16), hPMS1 (2q32) and hPMS2 (7p22) loci. Chromosomal regions 3p21 and 5q11-q13 were found hemizygously deleted in 46% and 23% of patients respectively. Half of the patients deleted at hMLH1 were also deleted at hMSH3. The shortest regions of overlapping (SRO) deletions were delimited by markers D3S1298 and D3S1266 at 3p21 and by D5S647 and D5S418 at 5q11-q13. Currently, the genes hMLH1 (3p21) and hMSH3 (5q11-q13) are the only known candidates located within these regions. The consequence of these allelic losses is still unclear because none of the breast cancers examined displayed microsatellite instability, a hallmark of mismatch-repair defect during replication error correction. We suggest that hMLH1 and hMSH3 could be involved in breast tumorigenesis through cellular functions other than replication error correction.