The occurrence of rare hypervariable Ha-ras alleles or of a rare c-mos allele in white blood cell DNA is claimed to be associated with susceptibility to cancer. We analyzed a group of patients with intracranial tumors to determine whether the occurrence of rare alleles at the Ha-ras locus and at the c-mos locus was increased in comparison with normal individuals. We found a higher incidence of rare hypervariable Ha-ras alleles (9.5% to 3%) and a 5 kilobase EcoRI c-mos allele (2.5% to 0%) in the patients. These results are consistent with the opinion that such unusual alleles are associated with a predisposition to intracranial tumors.