The present decade marks the end of a unique generation--when the public benefited from the prevention of mental retardation phenylketonuria (PKU) without having to deal fully with the problems which can be anticipated from the reproduction of successfully treated, phenylketonuric individuals. The dysgenic effect (increase in prevalence of the PKU gene) from this reproduction will have negligible influence on the frequency of mental retardation from PKU over the next few centuries. In contrast, a dysgenic effect from maternal PKU will cause a rebound in frequency of mental retardation within this decade. Failure of the PKU programs as a result of maternal PKU, could affect the public attitude toward programs for the study and prevention of mental retardation and genetic diseases. Minimizing and explaining the rebound in frequency of mental retardation will be a difficult but necessary task for workers in the field of mental retardation.