We report a newly recognized bone disorder consisting of polyostotic expansile osteolysis affecting long bones and iliac bones; hyperostosis of the skull, thoracic cage, and medial portion of both clavicles; pectus carinatum; gigantiform synovial masses of the elbows and knees; atrial septal defect; cardiomegaly; unilateral cryptorchidism; and mental deficiency. Affected bones can be grouped into four general types of skeletal pathology: (1) expansile osteolysis, (2) osteolysis without expansion, (3) expansion without osteolysis, and (4) hyperostosis. Some bones remained unaffected. We have named the condition "polyosteolysis/hyperostosis syndrome." It is clearly at variance with any previously reported bone disorder, including familial expansile osteolysis, juvenile Paget disease, and McCune-Albright syndrome (and polyostotic fibrous dysplasia). Because our patient shared some features in common with juvenile Paget disease, we thought that mutational analysis of TNFRSF11B was indicated, even though our patient had some manifestations not found in juvenile Paget disease. Direct sequencing failed to identify a TNFRSF11B mutation. Because the parents of our propositus were first cousins suggests that polyosteolysis/hyperostosis syndrome may possibly have autosomal recessive inheritance.