Carnosinase deficiency presented as a progressive neurological problem with sensory polyneuropathy in a 12-year-old male. Carnosinuria was present, even on a meat-free diet, although carnosinemia was not observed. An increased amount of unmetabolized carnosine was found in the urine after a carnosine load. Serum and tissue (liver and nerve) from the patient showed deficient carnosinase activity (Fleisher et al. 1978, 1980). Morphometric and fine structural studies on the nerve and skin biopsies are presented, as is a review of the literature on carnosinase deficiency in mentally retarded patients.