Combined 1p/19q deletions are very prevalent in oligodendrogliomas (OGs) and, to a lesser extent, in oligoastrocytomas (OAs). These losses are associated with responsiveness to therapy. Using array-based comparative genomic hybridization, we screened for recurrent genomic alterations in OG and oligoastrocytoma subtypes on chromosome 19. Concomitant 1p/19q loss was detected in most of the tumors with allelic loss, but array-based comparative genomic hybridization revealed some tumors to have unrelated 1p/19q arm losses, suggesting alternative mechanisms of loss to that related to the reported t(1;19) translocation. Analyses of 1p/19q loss by fluorescence in situ hybridization and loss of heterozygosity assays and correlations of genomic data with the Ki-67 proliferation marker were also performed. Four 1q (or 19p) and 2 1p (or 19q) fluorescence in situ hybridization probe signals together with homozygosity of the 1p/19q microsatellites suggested a hypothetical mechanism of genome duplication consecutive to the loss of the derivative chromosome der(1p;19q) from the t(1;19)(1q;19p) translocation. This genome duplication was frequent in high-grade OGs and was strongly correlated with Ki-67 expression; thus, it could be related to tumor progression. Finally, in addition to the frequent 1p/19q loss, we report a novel 17q amplified region in OGs with BIRC5 as one of the possible candidate target genes of the amplicon.