We describe an attempt to measure efficacy of treatment in the Mendelian diseases of man. We used the McKusick Catalogs to identify 351 single gene diseases. We scored the impact of each disease in seven phenotypic categories: lifespan, reproductive capability, somatic growth, intellectual development, learning ability, capacity to work, and cosmetic effect. We then scored the success of treatment in ameliorating each of these component manifestations separately and together. The response to treatment was slight in the whole sample (n = 351): lifespan was increased in 15%, reproductive capability in 11%, and social adaptation in 6%. We observed that the mutant gene product was known in only 15% of the conditions comprising our sample. Since the mutant polypeptide is known in most inborn errors of metabolism, the diseases of this type (n = 65) in our sample of Mendelian traits were studied separately. In each of the seven categories of phenotypic impact, only a few of the hereditary metabolic diseases responded in any degree to specific treatment: the treatment gave complete relief in 12%, there was a partial response in 40%, and none in the remaining 48%. These findings have implications for prognosis, genetic counseling, and medical care of patients with Mendelian disease.