Glomerular filtration rate (GFR) is often used to evaluate and manage patients with renal disease. Few studies in children have compared accurate estimations of GFR (clearance of inulin and/or iothalamate) with the clearance of creatinine which, because of simplicity, has been used as an approximation of GFR. At reduced levels of GFR, studies in adults suggest that the mean of the creatinine and urea clearances closely approximate the GFR. The present investigation shows that the clearance of creatinine approximates the GFR at normal levels; however, at reduced levels the creatinine clearance and the mean of the creatinine and urea clearances both overestimate the GFR as measured by iothalamate. The clearance of creatinine remains a useful clinical tool if its limitations at low levels of GFR are realized.