A pregnant woman whose previous child had a diagnosis of I-cell disease was referred for evaluation of the fetus. Fluid obtained by amniocentesis and maternal serum showed abnormally increased levels of lysosomal enzymes suggesting that the fetus had I-cell disease. Sonography at 18 weeks showed abnormally short femurs and intrauterine growth retardation. The pregnancy was electively terminated at 19 weeks' gestation and the diagnosis was confirmed. Radiographs of the fetus demonstrated that the bony dysplasia is present early in fetal life with diffuse decrease in bone mineralization, a coarse, lacy, trabecular pattern, overall shortening and under-modelling of the long bones, subperiosteal bone deficiency in the diaphysis giving the appearance of periosteal new bone, hypoplasia of the anterior superior aspect of the upper lumbar vertebral bodies, broad ribs, abnormal pelvis with squared iliac wings and flattened acetabular roofs, and a small irregular calcaneal ossification center. There was good correlation between the radiographic findings and the microscopic findings in the bones. We observed deficient endosteal bone formation, small epiphyses, and poorly developed intervertebral discs. We speculate that this indicates impaired production of extra-cellular matrix by several different types of specialized mesenchymal cells. Abnormalities of transport of glycoproteins other than lysosomal enzymes or excess of extracellular acid hydrolases may be involved in the pathogenesis.