Background Very few studies have been published that compare plasma clearance of iohexol (Cio) with renal clearance of inulin (Cin). Study Design Diagnostic test study. Setting & Participants 60 children aged 11.6 ± 4.5 years with different kidney disorders were investigated. Index Test Plasma Cio calculated from the slope and a single point. Reference Test Renal Cin with continuous infusion during water diuresis. Results were compared with the correlation coefficients, bias and precision, accuracy percentage, root mean square error, and intraclass correlation. Other Measurements Measured creatinine clearance and estimated glomerular filtration rate based on serum creatinine level and height. Results Mean Cin was 70.7 ± 41.3 (SD) mL/min/1.73 m 2. Mean differences between Cio and Cin were 2.65 and 2.00 mL/min/1.73 m 2 for the slope and single-point methods, respectively. Precision was ±16 mL/min/1.73 m 2 and intraclass correlation was 0.92 in both methods. Proportions of Cio within 30% of Cin were 83.3% and 86.7% for the slope and single-point methods, respectively. Limitations A limited number of patients; no adults were studied. Conclusions Plasma Cio shows good agreement with renal Cin.