Oto-palato-digital syndrome type II (OPD II) is a lethal X-linked skeletal dysplasia with pleiotropic manifestations. The basic defect is not known. There has been only one detailed report of the chondro-osseous abnormalities in this condition describing abnormal periosteal ossification in a single case [1990: Am J Med Genet 36:226-231]. We report on three cases of OPD II emphasizing the chondro-osseous morphology. Although endochondral ossification was normal, periosteal ossification was defective with islands of cortical bone aplasia and hyperplasia of the periosteum. The trabecular bone was also extremely poorly formed and markedly hypercellular. Both membranous ossification and bone remodeling appear to be defective in OPD II and should account for part of the observed phenotype. The biglycan gene maps to Xq28 and is involved in bone formation, but was excluded as a candidate by direct sequencing of cDNA in one case.