BACKGROUND/AIMS: Cysteine dioxygenase (CDO, EC 188.8.131.52) catalyses the conversion of cysteine to cysteine sulphinic acid and controls the rate-limiting step of sulphate production. Many neurological and non-neurological diseases are associated with abnormalities in CDO activity, giving rise to reduced availability of sulphate. The importance of the kidney in the sulphation of xenobiotics has long been recognised, but little is known about the renal expression of key enzymes in this pathway. In order to address this, this report demonstrates the expression of CDO in the kidney. METHODS: Two previously characterised antibodies were used to investigate the localisation and expression of CDO using immunohistochemistry, in-situ hybridisation and Western blotting. RESULTS: Renal CDO was shown to exist as a 68-kDa protein, which was unaffected by levels of cysteine and methionine that had been previously shown to induce hepatic CDO. CDO protein expression was present in the proximal convoluted tubules of the cortex and the collecting ducts of both the medulla and papilla. DISCUSSION: These results suggest that renal CDO is immunologically identical to that of the liver. Its expression in the kidney tubules, the major site of sulphation in the kidney, suggests that CDO in the kidney may play a role in both xenobiotic metabolism and sodium and water homeostasis.