Urate, a naturally occurring product of purine metabolism, is a scavenger of biological oxidants implicated in numerous disease processes^,,, as demonstrated by its capacity of neuroprotection^,. It is present at higher levels in human blood (200–500 µM) than in other mammals, because humans have an effective renal urate reabsorption system, despite their evolutionary loss of hepatic uricase by mutational silencing^,,. The molecular basis for urate handling in the human kidney remains unclear because of difficulties in understanding diverse urate transport systems and species differences^,,. Here we identify the long-hypothesized^,, urate transporter in the human kidney (URAT1, encoded by SLC22A12), a urate–anion exchanger regulating blood urate levels and targeted by uricosuric and antiuricosuric agents (which affect excretion of uric acid). Moreover, we provide evidence that patients with idiopathic renal hypouricaemia (lack of blood uric acid) have defects in SLC22A12. Identification of URAT1 should provide insights into the nature of urate homeostasis, as well as lead to the development of better agents against hyperuricaemia, a disadvantage concomitant with human evolution.