Disorders of purine and pyrimidine metabolism are due to abnormalities in the biosynthesis, interconversion, and degradation of the purines—adenine and guanine—and of the pyrimidines—cytosine, thymine, and uracil. The purine nucleotides, their cyclic derivatives (cAMP and cGMP), and their more highly phosphorylated derivatives have functions in many aspects of intermediary metabolism. Purine compounds also function as signal transducers, neurotransmitters, vasodilators, and mediators of platelet aggregation. Disorders of purine metabolism—the end point of purine metabolism in humans is uric acid. When uric acid levels become supersaturated in body fluids, uric acid and sodium urate monohydrate crystallize, causing gout. This results from either overproduction or underexcretion of urate, or from a combination of these defects. Decreased net tubular urate secretion is most often due to genetic polymorphism in uric acid transporters and is the commonest cause of primary (‘idiopathic’) gout. Gout may be secondary to a wide variety of renal disorders. Gout is also a consequence of enzymatic defects that accelerate de novo purine synthesis. Acute attacks of gout are treated with nonsteroidal anti-inflammatory drugs, colchicine, or steroids. Hypouricaemia may be caused by inherited disorders of uric acid biosynthesis or may be due to inherited or acquired renal tubule transport defects. Disorders of pyrimidine metabolism—the de novo synthesis of pyrimidine nucleotides involves a series of six reactions beginning with the formation of carbamyl phosphate and concluding with orotidine monophosphate, which then undergoes a series of interconversion and salvage reactions. The inherited disorders of pyrimidine metabolism, which can present in a wide variety of ways, are much less common and/or much less easily recognized than disorders of purine metabolism.