Five different methods of assessment of glomerular filtration rate (GFR) were compared in 500 children older than one year of age. Results of each method were compared with standard inulin clearance values: 1. An evaluation of glomerular function may be inferred from plasma creatinine levels; the validity of this inference is, however, limited because of the wide range of normal values; 2. creatinine clearance values calculated over 3 hours significantly overestimate inulin clearance values; 3. clearance values derived from plasma creatinine concentration ahd height show a poor correlation with traditionally calculated inulin clearances; 4. clearance values calculated as the sum of twice the creatinine clearance and once the urea clearance, divided by three, better predict inulin clearance than either C-creatinine or C-urea alone; 5 inulin clearance values calculated by dividing the quantity of inulin perfused by its plasma concentration, once it has become stable, differ significantly from the values obtained by the standard inulin clearance.--Of all the methods studied, none was found to be an ideal alternative to the traditional inulin clearance when a precise assessment of GFR is warranted. It is concluded that in most clinical conditions derived clearance values do not have significant advantages over the simple determination of plasma creatinine, a valuable but not infallible index of glomerular function.