Molecular genetic analysis of several common cancers has yielded information concerning their etiology and prognosis. Restriction fragment length polymorphism (RFLP) studies showing consistent loss of specific DNA sequences in tumor tissue have identified genes, known as tumor suppressors, associated with the etiology of neoplasms of the adult colon, breast, lung, and brain. For several of these tumors, identifiable mutations of these genes in association with multiple chromosomal losses have been linked to a poor clinical outcome. Using RFLP and other techniques, we have determined that one or more tumor suppressor genes on chromosome 17p is involved in the etiology of both medulloblastoma and astrocytoma in children. Loss of chromosome 17p sequences is indicative of prognosis negative for these children. Molecular genetic data therefore supplement and in some cases surpass clinical criteria in predicting prognosis for these childhood tumors.