We have used the polymerase chain reaction and single strand conformation polymorphism (SSCP) methods to analyse the COL10A1 gene, which encodes collagen type X, in DNA samples from patients with metaphyseal dysplasia type Schmid (SMCD) and other related forms of metaphyseal dysplasia. Five cases of SMCD were sporadic and three others were familial. Abnormal SSCP profiles were observed in six instances. In two families, the altered pattern segregated with the phenotype. The heterozygous mutations corresponded to a glycine substitution by glutamic acid at position 595 and to an asparagine substitution by lysine at position 617. In one sporadic case, the sequence studies demonstrated that the individual was heterozygous for a single base deletion (del T 1908) that produced a premature stop codon. Three additional mutations were single base substitutions that affected highly conserved residues at positions 597, 644 and 648. In two additional individuals with SMCD, in two patients with unclassifiable forms of metaphyseal dysplasia, and in one family with epiphyso-metaphyseal dysplasia, SSCP analysis detected neutral polymorphisms in the entire coding sequence of the gene but no mutations. Our results demonstrate that mutations in the carboxy-terminal region of collagen X are specific for the SMCD phenotype. Mutations appear to be clustered into three small subdomains: one of them is rich an aromatic residues, the second includes the putative N-linked oligosaccharide attachment site and the third contains mostly hydrophilic residues. The absence of clinical variability between patients carrying heterozygous single base substitutions or small deletions suggests that, in both instances, the mutant collagen chains either fail to be incorporated into stable trimers or disturb type X collagen assembly.