We report a family with what appears to be a unique X-linked recessive disorder of isolated hyperostosis of the calvarium. Although irregularity of the calvarium and exophytic prominences of the frontoparietal bones were apparent in infancy, premature cranial suture closure did not occur and there was no evidence of increased intracranial pressure despite a Luckenshadel appearance of the skull. Other membranous bones and the tubular bones were not involved. Calvarial bone biopsy from one patient showed vacuolated histiocytes suggesting a storage disease; however, neurologic deterioration, hepatosplenomegaly, and dysostosis multiplex did not occur. The affected family members had normal stature, normal occipitofrontal circumference, and no other medical problems. The biochemical basis of this disorder is not known. Although storage of abnormal material is possible, the long-term prognosis seems favorable.