Osteosarcomas (OS) are aggressive tumors of the bone and often have a poor prognosis. The tumors exhibit karyotypes with a high degree of complexity, which has made it difficult to determine whether any recurrent chromosomal aberrations characterize OS. To address inherent difficulties associated with classical cytogenetic analysis, comparative genomic hybridization (CGH) was applied to OS tissue. Forty-one pediatric OS specimens were analyzed by a CGH technique: 24 female and 17 male patients, with a median age of 12 years and 4 months. Chromosomal abnormalities were highly diverse and variable, including gains of chromosome 1p, 2p, 3q, 5q, 5p, and 6p and losses of 14q (50% in 14q11.2), 15q, and 16p. A high level of losses of chromosome 21 was present (26/41 cases; P = 0.008), most often loss of the 21q11.2 approximately 21 region. These novel findings in chromosome 21 of pediatric OS tumors suggest that specific sequences mapping to these chromosomal regions are likely to play a role in the development of OS.