The characteristics of urate metabolism in renal hypouricemic patients with hematuria were studied to clarify the risk factors for hematuria in patients with renal hypouricemia. In 16 Japanese patients with isolated renal hypouricemia, urate metabolism was measured using the urate clearance study and the subtype of renal hypouricemia [defective presecretory reabsorption (Pre), defective postsecretory reabsorption (Post), enhanced tubular secretion (Secretion) and defective presecretory and postsecretory reabsorption (Pre&Post)] were determined by the pharmacological tests. Hematuria was seen in 7 out of the 16 patients (44%), all of whom were females (58%). Serum urate and urinary urate concentrations were significantly higher in the group with hematuria (Sur = 1.76 +/- 0.31 mg/dl and Uur/Ucr = 0.75 +/- 0.12: p<0.05) than in the group without hematuria (Sur = 1.44 +/- 0.46 mg/dl and Uur/Ucr = 0.56 +/- 0.04), although there was no difference in the urate excretion rate between the two groups. Hematuria was more likely to be accompanied by Post (75%) and Secretion (75%), which showed significantly higher urinary urate concentration (Uur/Ucr = 0.75 +/- 0.1 and 0.69 +/- 0.13, respectively) than by Pre (25%) and Pre&Post (0%), which showed lower urinary urate concentration (0.61 +/- 0.06 and 0.62 +/- 0.05, respectively). The risk factors for hematuria in patients with renal hypouricemia are the elevation of urinary urate concentration and the subtypes of Post and Secretion.