The existence of acute lymphoblastic leukemia (ALL) and osteosarcoma is described. An 8-year-old girl had osteosarcoma diagnosed on radiologic and pathologic examination during ALL maintenance treatment. Cytogenetic analyses in primary cell culture of osteosarcoma tissue from the patient showed complex chromosomal abnormalities including t(1;19), usually seen in B precursor cell ALL, and del 13, found in a great majority of primary osteosarcomas. To show the possibility of the existence of the genetic susceptibility caused by gene rearrangements, we used molecular technique. But we could not determine any association between gene and genetic susceptibility.