Chondro-osseous tissue from five patients with diastropic dwarfism was studied by histologie, histochcmical, and electron microscopic methods. The major abnormalities observed were: 1) irregular distribution of chondrocytes undergoing degeneration in the resting cartilage; 2) abnormal distribution of collagen in the resting cartilage; 3) a spectrum of fibrous matrix lesions in the resting cartilage which ranged from focal areas of aggregated collagen fibrils to large cystic lesions in which intracartilagenous ossification occurred; and 4) shortened, irregular cellular columns within the growth plate which were occasionally disrupted by matrix lesions extending from the resting cartilage. These alterations in chondro-osseous morphology have not been observed in any of the other skeletal dysplasias examined to date and appear to be pathognomonic for this disorder.Speculation The basic abnormality in diastropic dwarfism may be a metabolic abnormality in the chondrocyte which predisposes it to cell death or a processing defect in its synthesis of either collagen or proteoglycan. An enzyme deficiency could produce either type of defect and would be consistent with the autosomal recessive inheritance of this disorder.