The current World Health Organization (WHO) classification groups together both infratentorial neoplasms (medulloblastomas) and their supratentorial counterparts as primitive neuroectodermal tumors (PNETs), implying a common origin. Previous analyses of medulloblastoma have shown loss of chromosome arm 17p as the most frequent genetic abnormality: the molecular genetic constitution of supratentorial PNETS has not been systematically studied. We therefore examined 8 hemispheric PNETs and 35 medulloblastomas with 17p restriction fragment length polymorphism (RFLP) and microsatellite markers. We also examined the TP53 tumor suppressor gene by a combined polymerase chain reaction-denaturing gradient gel (PCR-DGGE) technique. Our results showed that all of the 17p markers tested were preserved in all of the supratentorial PNET specimens. In contrast, loss of distal chromosome arm 17p was detected in 37% of the medulloblastomas. Analysis of the TP53 gene showed 2 mutations in the medulloblastomas and no mutations in the supratentorial tumors. These results show that the most common molecular genetic abnormality in infratentorial PNETS is absent in their supratentorial counterparts and suggests that alternative pathways and genetic events may be involved in their etiology.