The choice of the optimal method for the measurement of renal function is based on the accuracy and the precision of the technique. The plasma clearance of nonradioactive iohexol has been proposed as a reliable alternative to renal clearance of inulin for estimation of GFR. However, the precision of this method in estimating GFR in patients with renal disease has not been determined so far. This issue was assessed by determining plasma clearance of iohexol on three different occasions during a 12-d period in 24 patients with renal disease and a wide range of renal function (creatinine clearance: 14 to 104 ml/min per 1.73 m2). Overall, the mean intraindividual coefficient of variation was 5.59%, and the reproducibility was 6.28%. The precision of the method also applied to the subgroup of patients with moderate-to-severe renal insufficiency, because a low coefficient of variation (5.71%) and a high reproducibility (6.57%) were found in patients with GFR < or =40 ml/min per 1.73 m2. It was also shown that the precision of GFR measurement by the plasma clearance of iohexol is not affected by the gender. These findings indicate that the method of plasma clearance of iohexol allows a good precision in the estimation of GFR in patients with normal renal function and different degrees of renal dysfunction.