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