Methylation of the promoter regions of CpG-rich sites in genes is the major mechanism for the silencing of many genes in tumors. Methylation of the key apoptosis-related gene caspase 8 (CASP8) has been reported in some childhood tumors and in neuroendocrine lung tumors. We examined the methylation status of 181 pediatric tumors and found frequent methylation in rhabdomyosarcomas (83%), medulloblastomas (81%), retinoblastomas (59%), and neuroblastomas (52%). Methylation frequencies were low in Wilms' tumors (19%) and absent in hepatoblastomas, acute leukemias, osteosarcomas, Ewing's sarcomas, and ganglioneuromas and in normal tissues. Methylation of CASP8 and the tumor suppressor gene RASSF1A were highly significantly correlated in all tumor types by both the chi(2) and the Fisher's exact tests (P < 0.0001 for both tests). Because the region of the gene examined by us and others is not located in the promoter region and lacks features of a CpG island, we explored the relationship between methylation and gene silencing in detail using 23 pediatric tumor cell lines. Studies included relating the methylation of the region to gene expression at mRNA and protein levels, enzymatic assays of gene function, clonal analysis of PCR amplicons of the region, and exposure to a demethylating agent. These studies indicated that methylation correlated with the loss of gene function in most cases; however, other mechanisms of gene inactivation were present in some cases. Posttranscriptional inactivation of the closely related gene caspase 10 was present in many cell lines. Our results suggest that deregulation of the death receptor pathway to apoptosis is frequent in many types of pediatric tumors and their cell lines.