A new sclerosing bone disease in two Japanese siblings born to first-degree cousin parents is reported. Clinically the disease is characterized by early developmental delay, hypotonia and later spastic paraplegia. The unique radiographic changes consist of peripheral osteosclerosis affecting predominantly metaphyses of the long bones and to a lesser degree ends of the ribs and clavicles, iliac crests, acetabulae, ischio-pubic synchondroses and vertebrae. The epiphyses are sclerotic in early life. The round bones, short tubular bones and the skull are little affected. The shafts of the tubular bones are osteopenic. Increased serum alkaline phosphatase was the only laboratory abnormality detected. We suggest the name "osteosclerotic metaphyseal dysplasia" for this disorder.