We examined alterations of the p16INK4, p14ARF, p15, TP53, and MDM2 genes in 30 osteosarcomas and 24 Ewing sarcomas. Among 21 osteosarcomas and 24 Ewing sarcomas, p16INK4, p14ARF, and p15 abnormalities were found in 4 (19%), 2 (9%), and 3 (14%) osteosarcomas, respectively, and in 4 (17%), 3 (13%), and 4 (17%) Ewing sarcomas, respectively. The alterations of p16INK4, p14ARF, and p15 included homozygous deletions spanning all 3 genes, methylation of p16INK4 or p15, and a nonsense mutation of p16INK4, which simultaneously caused a missense mutation of p14ARF. Alterations of TP53 were found in 15 (50%) of 30 osteosarcomas and 1 (3%) of 24 Ewing sarcomas. None of the sarcomas showed MDM2 amplification. While TP53 abnormalities were far more frequent in osteosarcoma than in Ewing sarcoma, alterations of p16INK4, p14ARF, and p15 were present at similar frequencies in the two types of sarcoma. The event-free survival (EFS) was worse in Ewing sarcoma patients with p16INK4 and p14ARF mutation/deletion than in those without the mutation/deletion (P = 0.019), and EFS was worse in osteosarcoma patients with TP53 alterations than in those without TP53 alterations (P = 0.048). The different incidence of TP53 abnormalities in the 2 types of sarcoma may reflect differences of the molecular processes through which the 2 types of tumor develop.