Hereditary multiple exostoses (EXT; MIM 133700) is an autosomal dominant condition characterized by growth of multiple benign cartilage-capped tumors. EXT greatly increases the relative risk to develop chondrosarcoma, although most chondrosarcomas are sporadic. This observation suggests that, like the genes responsible for retinoblastoma and other dominantly inherited cancer susceptibility disorders, the genes that cause EXT may have tumor-suppressor function and may play a role in the pathogenesis of the related sporadic tumors. To investigate this hypothesis, we evaluated chondrosarcomas for loss of constitutional heterozygosity (LOH) at polymorphic loci linked to three recently identified genomic regions containing genes involved in EXT. LOH for markers linked to EXT1 on chromosome 8 was detected in a chondrosarcoma that arose in a man with EXT. Four of 17 sporadic tumors showed LOH for markers linked to EXT1, and 7 showed LOH for markers linked to EXT2 on chromosome 11. In all, LOH was observed for markers linked to EXT1 or EXT2 in 44% of the 18 tumors, whereas heterozygosity was retained for markers on 19p linked to EXT3. These findings support the hypothesis that genes on 8q and the pericentromeric region of 11 have tumor-suppressor function and play a role in the development of chondrosarcomas.