DNA methyltransferases (DNMTs) 1, 3a, and 3b affect DNA promoter methylation; studies have suggested that they have important roles in the development of cancers. In this study, we analyzed the expression of DNMTs 1, 3a, and 3b; the MIB-1 labeling index; and their clinical significance in 6 normal retinas and 62 retinoblastomas using immunohistochemical analysis. We found that DNMT proteins were not expressed in normal retinas, whereas they were frequently expressed in retinoblastomas (DNMT1, 100%; DNMT3a, 98%; and DNMT3b, 92%). Compared with well-differentiated retinoblastomas, the expression of DNMTs 1 and 3a significantly increased in poorly differentiated retinoblastomas (P = .002 and P = .003, respectively); in addition, the frequency of their increased expression was high. DNMT1 expression was significantly higher in invasive retinoblastoma. Furthermore, the expression of DNMTs was positively correlated with the MIB-1 labeling index in retinoblastoma. Our findings suggest that the overexpression of DNMTs 1, 3a, and 3b may be related to retinoblastoma tumorigenesis and progression and may represent a novel approach for retinoblastoma therapy.