Three patients with the syndrome of inappropriate secretion of antidiuretic hormone had elevated uric acid clearances. Their uric acid clearances decreased markedly after the administration of pyrazinamide. Probenecid was given to two of them and it produced large increases in uric acid clearance. These data suggest that enhanced secretion in the renal tubules was responsible for the increased clearance of uric acid. This article provides evidence that hypouricemia in the syndrome of inappropriate secretion of antidiuretic hormone is due to increased tubular urate secretion.