A form of osteopetrosis is described in two sibs which differs from the well-established autosomal dominant and recessive forms. The clinical course is mild with no evidence of clinically important encroachment upon the bone marrow or cranial nerve foramina; the skeletal radiographs show a mild generalized increase in bone density and metaphyseal modeling defects affecting primarily the distal femurs. The inheritance pattern is compatible with either an autosomal or X-linked recessive mode. The histologic features are distinct as well, and suggest that the pathogenesis of the condition differs from that of the usual forms of osteopetrosis.