Ewing's sarcoma, peripheral primitive neuroectodermal tumors, and Askin tumors are referred to as Ewing tumors (ETs), and are characterized by high MIC2 expression and a t(11;22)(q24;q12) or other rearrangements involving 22q12. In addition to these constant aberrations, facultative numerical and structural aberrations have been reported: gains of chromosomes 8 and 12, the unbalanced translocation t(1;16), and deletions at the short arm of chromosome 1. To evaluate the frequency and to study the biological impact of these facultative aberrations, we analyzed tumor specimens from 58 ET patients by classical cytogenetics and/or in situ hybridization techniques and compared these data with clinical parameters. Gains of chromosomes 8 and 12 were detected in 55% (32/58) and 24% (14/58) of the cases, respectively. Loss of chromosome 16 or der(16)t(1;16) chromosomes were found in 20% (10/51); deletions at 1p36 were observed in 18% (9/51) of the cases evaluated. The presence of these aberrations did not correlate with age and sex of the patients, with the location of the primary tumor or with the extent of disease at diagnosis by chi-square analysis and Fisher's exact test. Patients with tumors harboring gains of chromosome 8 showed a slightly better clinical outcome (n ϭ 14/30, P ϭ 0.17), whereas gains of chromosome 12 did not influence the clinical outcome (n ϭ 7/30, P ϭ 0.63). However, Kaplan and Meier analysis revealed that deletions at the short arm of chromosome 1 were associated with an unfavorable outcome in patients with localized disease (n ϭ 6/22; P ϭ 0.004).