We have constructed a physical map covering over 4 Mb of human chromosome 8q24.1 and used this map to refine the locations of the genes responsible for Langer-Giedion syndrome. The map is composed of overlapping YAC clones that were identified and ordered in relation to sequence tagged sites mapped to the Langer-Giedion chromosomal region on somatic cell hybrids. The minimal region of overlap of Langer-Giedion syndrome deletions, previously identified by analysis of 15 patients, was placed on the map by analysis of 2 patients whose deletions define the endpoints. The chromosome 8 breakpoint of a balanced t(8;9)(q24.11;q33.3) translocation from a patient with trichorhinophalangeal syndrome (TRPS I) was found to be located just within the proximal end of the minimal deletion region. A deletion of 8q24.11-q24.3 in a patient with multiple exostoses was found to overlap the distal end of the LGS deletion region, indicating that the EXT1 gene is distal to the TRPS1 gene and supporting the hypothesis that Langer-Giedion syndrome is due to loss of functional copies of both the TRPS1 and the EXT1 genes.