Retinoblastoma is a relatively rare disease, and one which is fraught with tragedy for both parents and patients. A study was made on the population of Ohio on the following points: (1) Did the mode of inheritance as exhibited in Ohio families confirm the findings of other workers that the disease was caused by a dominant incompletely penetrant gene? (2) What estimate could be made of the degree of penetrance? (3) What was the estimate of the mutation rate of the normal gene to one for retinoblastoma? (4) What was the frequency of the disease among the births in Ohio? Material for the Study The names of all the children who had been born in 1940 through 1956, and who had developed retinoblastoma by the end of 1958 were asked for, the requests being sent (1) to all registered ophthalmologists in Ohio, (2) to all hospitals apart from mental and