Osteosarcoma was detected in two siblings. Their human leukocyte antigen (HLA) phenotypes were completely identical, although they were different from those of osteosarcoma patients in previous reports. Despite an extensive search of family and past history, no significant background related to the induction of cancer could be found. These cases suggest that genetic similarity may influence the development of osteosarcoma. Ascertainment of the HLA phenotypes in siblings with osteosarcoma might be a useful strategy to facilitate the early diagnosis of this tumor.