Recurrent, site-specific chromosome translocations and other cytogenetic abnormalities are being described in ever-increasing numbers and types of human tumors. Primary brain tumors are the most common pediatric solid tumor and differ from those of adults in both histology and clinical behavior. We examined chromosomes from 21 primary pediatric brain neoplasms grown in short-term tissue culture, including 6 astrocytomas, 10 primitive neuroectodermal tumors, and 5 other tumors. Karyotypes from 3 of 5 astrocytomas were abnormal, as were those of 9 of 10 primitive neuroectodermal tumors. Numerical abnormalities were found in 6 tumors and structural aberrations in 12 tumors. Deletions, additions, and translocations involving the short arm of chromosome 1 were observed in 5 tumors, with chromosome breakpoints ranging from 1p1 to 1p3. An isochromosome of the long arm of 17, i(17q) was the most frequent site-specific structural abnormality, found in 1 anaplastic astrocytoma and 2 recurrent cerebellar primitive neuroectodermal tumors, one with islands of anaplastic astrocytoma. These results differ from reported chromosome studies of adult brain tumors, suggesting that pediatric brain tumors may differ from those of adults when examined at the genetic level. Additional chromosomal and molecular studies of brain tumors from children are warranted to define these differences.