A case of familial polymorphous oligodendroglioma, occurring in a brother and sister, is presented. Polymorphous oligodendrogliomas have a characteristic histopathology consisting of scattered multinucleated giant cells against a typical oligodendroglial background. The oligodendroglial character of the tumors was underlined by positive immunostaining for antigalactocerebroside, anticarbonic anhydrase, and anti-leu-7, without expression of glial fibrillary acidic protein. Both tumors were immunopositive for p53, suggesting a mutation in the p53 gene. No incidence of cancer was recorded in the family. This is the first report of familial occurrence of this particular subtype of oligodendroglioma.