Pseudoachondroplasia (PSACH) is a well characterized dwarfing condition mapping to chromosome 19p12-13.1. Cartilage oligomeric matrix protein (COMP), a cartilage specific protein, maps to the same location within a contig that spans the PSACH locus. Using single strand conformation polymorphism (SSCP) analysis and nucleotide sequencing we have identified COMP mutations in eight familial and isolated PSACH cases. All mutations involve either a single base-pair change or a three base-pair deletion in exon 17B. Six mutations delete or change a well conserved aspartic acid residue within the calcium-binding type 3 repeats. These results demonstrate that mutations in the COMP gene cause pseudochondroplasia.