Pyrazolo[3,4-d]pyrimidines have a close structural resemblance to the substrates for the enzyme xanthine oxidase, hypoxanthine (6-hydroxypurine) and xanthine (2,6-dihydroxypurine). These compounds ar e capable of binding to the enzyme and strongly inhibit its activity. They have been studied for their role in the treatment and prophylaxis of hyperuricemia in human patients as well as potential an ti-tumor agents.