Parental age in sporadic hereditary retinoblastoma. Of 104 children with sporadic hereditary retinoblastoma born between 1945 and 1970, we studied the age of their parents at the birth and compared this age with the mean age of parents at the birth of their children during the same period in The Netherlands. The mean age of fathers at the birth of their children with sporadic hereditary retinoblastoma (33.7 years) was significantly higher than the mean age of fathers at the birth of their children in the general population (32.5 years) (P less than .05, one sided). Similarly, the mean age of mothers at the birth of their children with sporadic hereditary retinoblastoma (31.2 years) was significantly higher than the mean age of mothers at the birth of their children in the general population (29.5 years) (P less than .05, one sided). We further analyzed this parental age factor by measuring the relative risk of age groups and comparing the incidence of sporadic hereditary retinoblastoma in the various parental age groups with the incidence of sporadic hereditary retinoblastoma in the total population. Mothers 35 years of age or older had a relative risk of 1.7 to have a child with sporadic hereditary retinoblastoma compared with mothers in the population in general (P = .006, one sided). Similarly, fathers 50 years of age or older had a relative risk of 5.0 to have a child with sporadic hereditary retinoblastoma compared with fathers in the population in general (P = .04, one sided). No parental age effect was found in children with nonhereditary retinoblastoma. We conclude that a high paternal and a high maternal age are significant risk factors for sporadic hereditary retinoblastoma. 