January 2005. A boy aged 7 years was found to have a tumor arising from the roof of the fourth ventricle. Histopathologically, part of the tumor appeared as a PNET, while neuroglial tissue, striated and smooth muscle, cartilage and small glandular structures were present in other regions. Tumor cells in both primitive and mature elements showed a loss of chromosome 17p accompanied by a gain of 17q, a pattern consistent with the presence of an isochromosome 17q. This abnormality is not characteristic of intracranial germ cell tumors, but is present in over 30% of medulloblastomas. On the basis of the histologic and genetic abnormalities, we propose a diagnosis of PNET with multilineal differentiation.