The disorders of purine metabolism encompass a spectrum of clinical abnormalities. Striking features are the hyperuricemia, neurologic abnormalities and unusual behavior of Lesch-Nyhan disease. Renal stone disease and deafness characterize PRPP deficiency and xanthine oxidase deficiency, as well as in orotic aciduria. In purine nucleotide phosphorylase deficiency and adenosine deaminase deficiency, there is defective immune function. Patients with deficiency of adenylosuccinase have seizures and delayed development. Myoadenylate deaminase deficiency causes myopathy. Hypouricemia is found in xanthine oxidase deficiency, molybdenum cofactor deficiency and purine nucleoside phosphorylase deficiency. Deficiency of deoxyguanine kinase deficiency leads to mitochondrial DNA depletion and hepatic failure. Deficiency of pyrimidine 5'-nucleotidase leads to hemolytic anemia. Toxic responses to customary doses of 5-Fluorouracil are found in disorders of pyrimidine metabolism, particularly dihydroprimidine dehydrogenase deficiency.