We report on the genetic counseling and gene testing of patients with retinoblastoma who visited the National Cancer Center Hospital, Tokyo, from April 1997 through September 2003. During this period, 73 probands visited the clinic, and gene testing was performed in 51 individuals. Germline mutations of the RBI gene were detected in 20 individuals (39%); the frequencies were 82% (9/11) in bilateral/familial retinoblastoma, 50% (2/4) in unilateral/familial retinoblastoma, 50% (8/16) in bilateral/nonfamilial retinoblastoma, and 5% (1/20) in unilateral/nonfamilial retinoblastoma. Gene testing is indicated in the medical practice of hereditary retinoblastoma for familial risk assessment, while prior counseling is important for an understanding of the risks and benefits of gene testing. With improvements in patient prognosis, counseling for adult survivors is increasing in importance. Assessment of genetic risk to the offspring and prevention of secondary cancer are the primary issues of concern. Presymptomatic diagnosis of infants is effective for the proper assessment of the genetic risk and for making follow-up schedules for the detection of the tumor at an early stage.