Retinoblastoma is a tumor of the embryonic neural retina in young children. The DNA methyltransferase 1 (DNMT1) gene has been demonstrated to be transcriptionally activated in cells lacking retinoblastoma 1 (RB1). Thus, there is a direct interaction betweenDNMT1andRB1 in vivo. The present study hypothesized that uncontrolledDNMT1, DNMT2andDNMT3expression may lead to a high level of global genome methylation causing a second hit or where both alleles are altered, inRB1and/or inactivation of other genes in retinal cells. To test this, the global genome methylation levels were analyzed in 69 patients with retinoblastoma, as well as 26 healthy siblings and 18 healthy unrelated children as the control groups. Peripheral blood and tumor tissue samples were obtained from 32 patients. The expression levels ofDNMTgenes were also determined in cell lines. Based on the median levels of global genome methylation in patients, higher genome-wide methylation levels in peripheral blood were associated with a 3.33-fold increased risk for retinoblastoma in patients compared with all healthy controls (95% confidence interval, 0.98-11.35; P<0.0001). The level of global genome methylation and the expression ofDNMTgenes were increased in the WERI-RB-1 cell line, which has a mutatedRB1gene, compared with a wild-typeRB1-expressing cell line. These results supported the hypothesis that epigenetic alterations, as well as mutations inRB1, may be associated with the oncogenesis and inheritance of retinoblastoma. The repression of genes that interact withRB1, such as theDNMTgene family, may be important in patients with retinoblastoma with alterations inRB1, and may serve a role in the treatment and regression of retinoblastoma.