(1) Excretion of abnormally large amounts of xanthine in the urine. (2) A disorder [MIM*278300], characterized by urinary excretion of xanthine in place of uric acid, hypouricemia, and occasionally the formation of renal xanthine stones. There are two types: type I is due to xanthine dehydrogenase (XDH) deficiency, and type II is due to deficiencies of both XDH and aldehyde oxidase. Autosomal recessive inheritance, caused by mutation in the XDH gene on chromosome 2p in some cases. SYN: xanthiuria, xanthuria. [xanthine + G. ouron, urine]