Evidence is presented from the literature that there is little need to postulate delayed mutation for the retinoblastoma locus. Both penetrance and expressivity in the gene carrier can be defined as a variable determined by genetic and environmental factors, not by a Poisson distribution of tumors formed. Of individuals who received a new mutation from a healthy parent, approximately 13% do not manifest retinoblastoma, and the heritability of the host resistance is estimated at about 90%. The nonhereditary form of retinoblastoma may occur in the most susceptible group of the population.