A 27-year-old man was treated for an aggressive cerebellar medulloblastoma that, at operation, exhibited dural invasion. Six months after gross total resection and radiation therapy, a "surgical metastasis" developed in the lower portion of the surgical scar. The tumor grew rapidly down into the right side of his neck. Chemotherapy failed, and he subsequently died. Cytogenetic and molecular genetic studies revealed multiple numeric and structural chromosome abnormalities, including an abnormal chromosome 17p arm, more than 100-fold N-myc amplification, a rearranged c-myc gene, and a 16-base pair deletion involving exon 7 of the p53 gene. We postulate that these genetic features may have contributed to the aggressive behavior of the tumor.