Epigenetic models for tumor formation assume that oncogenic transformation results from changes in the activity of otherwise normal genes. Since gene activity can be inhibited by DNA methylation, and inactivation of tumor suppressor genes is a fundamental process in oncogenesis, we investigated the methylation status of the retinoblastoma suppressor gene (RB gene) on chromosome 13, in blood and tumor cells from 21 retinoblastoma patients. Using methylation-sensitive restriction enzymes and a cloned DNA probe for the unmethylated CpG island at the 5' end of RB gene, we obtained evidence of hypermethylation of this gene in a sporadic unilateral retinoblastoma tumor. The closely linked esterase D gene and a CpG-rich island on chromosome 15 were not affected. We suggest that changes in the methylation pattern of the RB gene play a role in the development and spontaneous regression of some retinoblastoma tumors.