Epigenetic inactivation of the RASSF1A tumor suppressor gene (TSG) at chromosome 3p21.3 was examined in medulloblastoma, the most common malignant brain tumor of childhood. Seventy-nine % (27 of 34) of primary tumors and 100% (8 of 8) of medulloblastoma cell lines displayed extensive tumor-specific DNA hypermethylation across the RASSF1A promoter-associated CpG island. Hypermethylation was associated with epigenetic silencing of RASSF1A transcription in medulloblastoma cell lines, and RASSF1A expression in these lines was restored after treatment with the DNA-methyltransferase inhibitor 5-aza-2'-deoxycytidine. No evidence was found of RASSF1A inactivation by genetic mechanisms (gene mutation or deletion) in either cases with no evidence of RASSF1A hypermethylation or paired normal/tumor cases and cell lines with evidence of total RASSF1A CpG island hypermethylation. Epigenetic inactivation by biallelic hypermethylation therefore represents the primary mechanism of RASSF1A gene inactivation in medulloblastoma. Furthermore, RASSF1A hypermethylation is a frequent event in medulloblastoma tumorigenesis detectable in adult (5 of 7) and pediatric patients (22 of 27) and in all histological variants and age and sex groupings. Importantly, these data demonstrate that comprehensive analysis of the genome and epigenome will be required for identification of the key tumor suppressor genes involved in medulloblastoma development.