Most of the primates, unlike other mammals, have mutations in urate oxidase gene and cannot catabolize urate in the bodies. In addition to the genetic defects, some human subjects have various abnormalities in urate metabolism. Urate metabolism abnormalities are classified into two categories, hyperuricemia and hypouricemia. Usually, the urate pool size of an adult male is about 1,200 mg, and 700 mg urate is produced daily. The production is balanced by the excretion of urate into urine (500 mg) and intestine (200 mg). If this balance is disturbed, either hyperuricemia or hypouricemia occurs. According to the mechanisms, hyperuricemia is classified into overproduction and underexcretion, and hypouricemia into underproduction and overexcretion. Overproduction of ruate is caused by PRPP synthetase superactivity, HPRT deficiency, leukemia and alcohol ingestion. Underexcretion of urate is caused by renal insufficiency and treatment by diuretics. Underproduction of urate is caused by xanthine dehydrogenase deficiency, purine nucleoside deficiency and allopurinol treatment. Overexcretion of urine is caused by familial renal hypouricemia, Fanconi's syndrome, diabetes mellitus and treatments with benzbromarone and probenecid. All of these conditions are classified, according to other aspects, into primary and secondary, and genetic and non-genetic abnormalities.