In the present cytogenetic analysis of 116 pediatric brain tumors, chromosomal abnormalities were demonstrated in 44 cases, 48 cases revealed only 46,XX or 46,XY cells, and 24 cases were nonproductive. In contrast to studies of adult brain tumors in which isolated loss of one X or the Y chromosome is often encountered, 45,X,-X and 45,X-Y stemlines or sidelines were not observed in this series of childhood tumors. Among the 17 medulloblastomas with cytogenetic abnormalities, chromosome 1 was most frequently affected by structural deviations; the most prevalent specific alteration (7 of 17 tumors) was loss of 17p, through i(17)(q10) or unbalanced translocation. The majority of low grade astrocytomas had normal stemlines, although one pilocytic astrocytoma and one cerebellar astrocytoma had frequent telomeric associations and a second pilocytic astrocytoma had a clone with trisomy 11. Thirteen of 19 high-grade and recurrent astrocytic tumors had abnormal stemlines that were approximately equally divided among cases with chromosomal counts in the near-diploid, hyperdiploid, and near-triploid-tetraploid ranges. Although no consistent abnormalities were observed, subsets of cases had structural abnormalities of chromosome 3, 7q, 9q, or 17p. The cases of childhood brain tumors described here demonstrate that 45,X,-X, and 45,X,-Y stemlines or sidelines are rare in these tumors and confirm frequent loss of 17p in medulloblastomas. High-grade astrocytic tumors in children frequently have abnormal stemlines, often in the hyperdiploid and polyploid ranges, and they differ from high-grade gliomas in the adult by lacking consistent numerical and structural deviations.