We report chromosome results from 108 pediatric central nervous system (CNS) tumors. From our data and those in the literature we found that (1) cerebellar and low-grade astrocytic tumors, including gangliogliomas, are most often karyotypically normal; (2) supratentorial tumors were more frequently high-grade tumors that demonstrated a complex karyotype. Chromosome abnormalities were similar to those described in adult astrocytic tumors, namely, +7, 9p abnormalities, and -10; (3) primitive neuroectodermal tumors (PNETs) were virtually always karyotypically abnormal with a high frequency of +7, -8, i(17q), and -22. PNETs with -22 may represent a subset of tumors; (4) typical choroid plexus papillomas showed a normal karyotype, atypical papillomas showed a hyperdiploid karyotype (with +7, +12, and +20), choroid plexus carcinomas showed a hyperhaploid karyotype; (5) a few ependymomas showed hyperdiploidy or hypertetraploidy; (6) germ cell tumors showed complicated karyotypes; (7) monosomy 22 or 22q abnormalities appear to be a recurring finding in the malignant rhabdoid tumors; and (8) meningiomas showed -22 or 22q abnormalities associated with a complex karyotype. In general, in pediatric CNS tumors the least differentiated neoplasms have the greatest number of cytogenetic abnormalities. However, our present morphologic criteria for tumor diagnosis do not always correlate with a consistent karyotype, and further study of pediatric brain tumor morphology, site, behavior, and karyotype is required.