Brain tumors are the most common solid tumors of childhood, accounting for over 20% of cancers in children under 15 years of age. Pilocytic astrocytomas (PAs), World Health Organization grade I, are one of the most frequently occurring childhood brain tumors, yet little is known about genetic changes characterizing this entity. We have used microarray comparative genomic hybridization at 0.97 Mb resolution to study a series of PAs (n = 44). No copy number abnormality was seen in 64% of cases at this resolution. However, whole chromosomal gain (median 5 chromosomes affected) occurred in 32% of tumors. The most frequently affected chromosomes were 5 and 7 (11 of 44 cases each) followed by 6, 11, 15, and 20 (greater than 10% of cases each). Findings were confirmed by fluorescence in situ hybridization and microsatellite analysis in a subset of tumors. Chromosomal gain was significantly more frequent in PAs from patients over 15 years old (p = 0.03, Fisher exact test). The number of chromosomes involved was also significantly greater in the older group (p = 0.02, Mann-Whitney U test). One case (2%) showed a region of gain on chromosome 3 and one (2%) a deletion on 6q as their sole abnormalities. This is the first genomewide study to show this nonrandom pattern of genetic alteration in pilocytic astrocytomas.