We welcome the comments and interest of Drs Sterner, Bäck, and Nyman1 regarding our study.2 We agree that until inulin is available again for studies of humans, it will be difficult to directly assess the comparable accuracy of iothalamate and iohexol in humans. Our study of 150 patients across a broad range of glomerular filtration rates (GFRs) showed a 15% difference in urinary clearance of iothalamate versus iohexol. Creatinine clearances were significantly greater than both iothalamate (by 24%) and iohexol (by 38%), suggesting that neither is likely to have a secretory component approaching creatinine.