Using 11 restriction fragment length polymorphism markers, we examined loss of heterozygosity on the long arm of chromosome 17, where one or more genes responsible for hereditary breast and ovarian cancers may be present, in sporadic forms of 94 ovarian and 246 breast cancers. Loss of heterozygosity was observed in 33 of 84 (39.3%) ovarian and in 88 of 214 (41.1%) breast cancers that were informative with at least one marker. Detailed deletion mapping of chromosome 17q in these cancers identified two distinct, commonly deleted regions. One was located between 17q12 and 17q21.3 and the other between 17q25.1 and 17q25.3. In breast cancers, the proximal commonly deleted region was between two loci defined by markers CI17-701 and CI17-730 at 17q21.3, which are 2.4 cM apart. This segment overlaps the region that includes the putative gene for hereditary breast and ovarian carcinomas. The results suggest that at least two tumor suppressor genes associated with sporadic ovarian and breast cancers are present on chromosome 17q and that one of them may be the same gene that is responsible for the hereditary form.