We report here a recurrent frameshift mutation within the carboxyl-terminal noncollagenous domain coding region of the type X collagen gene (COL10A1) in a Japanese patient with Schmid metaphyseal chondrodysplasia. The mutation involves deletion of a CT dinucleotide from position 1992 (1992delCT), and produces a frameshift which creates a premature termination codon close to the site of the deletion. The predicted length of the mutant polypeptide is 664 amino acids, which is shorter than the wild type polypeptide (680 amino acids). A 1992delCT mutation of COL10A1 has been previously reported in one family. The independent occurrence of de novo mutation of this specific dinucleotide repeat suggests that this region is a possible mutational hot spot on COL10A1.