We performed a cytogenetic study on 24 pilocytic astrocytomas: 23 in children and 1 in a young adult. We observed 12 normal karyotypes. In 12 karyotypes with structural and/or numerical abnormalities, chromosomes 7, 8, and 11 were most frequently involved. One case recurred and presented chromosomal abnormalities (hyperdiploidy) in the first tumor and additional structural abnormalities in the second tumor. We believe that chromosomal abnormalities in pilocytic astrocytomas are frequent and indicate tumoral progression.