We identified the chromosome 11q23 region as containing a putative tumour suppressor gene(s) frequently deleted in nonfamilial breast and other cancers. To define this region(s) further, we performed a systematic genetic analysis at chromosome 11q14-qterm in sporadic breast and colorectal cancer. Tumour and constitutional DNA from a panel of 81 cases (51 breast and 30 colorectal cancers) were analysed with multiple microsatellite markers distal to 11q13. Of 51 breast cancers, 31 of 49 informative cases (63%) showed LOH at the 11q22-q23.1 region (approximately 8 Mb). Furthermore, 23 of 45 informative cases (51%) had a deletion at 11q25 (approximately 2 Mb). Overall, LOH on 11q occurred in 37 of 51 breast cancers (72%). Colorectal cancers had LOH at 11q22 in two of 18 informative cases (11%), LOH at 11q23.3 in two of 17 informative cases (12%) and LOH at 11q25 in three of 20 informative cases (15%). Overall, LOH at 11q occurred in five of 30 colorectal cancers (16%). This data shows that chromosome 11q contains at least two independent regions (one novel) frequently deleted in breast cancers. Contrary to previous reports, LOH at distal 11q is not frequent in colorectal cancer. Chromosome 11q22-q23.1 and 11q25-qterm contain putative tumour suppressor genes with a significant role in breast but not colorectal carcinogenesis.