In this paper, two cases with mucolipidosis type II (I-cell disease) (proven in one presenting newborn and presumed in an elder deceased brother) are presented. These infants showed severe skeletal changes with diffuse periosteal new bone formation in long bones and ribs, marked osteopenia, and resorption of scapula, clavicula, and mandible. There was also irregular demineralization of metaphyses of long tubular bones, as seen in rickets. The activities of serum alkaline phosphatase and parathyroid hormone were markedly elevated. Phosphorus was decreased. Serum 1,25-dihydroxyvitamin D was slightly elevated, but 25-hydroxyvitamin D and calcium were normal. Dysostosis multiplex resembling rickets and very high alkaline phosphatase activity were due to defective osteoblastic activity, but the mechanism of elevated parathyroid hormone was not clear. We conclude that early skeletal manifestation of mucolipidosis type II is not clearly identified and that differentiation from congenital rickets or congenital hyperparathyroidism could be difficult. It is speculated that hyperparathyroidism in these patients could be related to the calcium-sensing receptor malfunction in the parathyroid gland.