BACKGROUND The purpose of presenting this case is to demonstrate the degree to which the kidney is capable of selectively excreting a massive load of sodium and water when challenged with both of these, without altering the plasma levels of other ions. CASE REPORT An 8-year-old boy was admitted in severe renal failure. Workup demonstrated a high grade obstruction of a single kidney. Following dialysis, the patient underwent surgery to correct the obstruction and he developed post-obstructive diuresis. Within one week he was receiving 34 liters of essentially 1/2 Na by IV and by mouth and was excreting 70% of his filtered load of water and 50% of his filtered load of sodium. As soon as the administered fluids and Na were cut back, the kidney responded appropriately. CONCLUSIONS While post-obstructive diuresis is a real phenomenon, very frequently it is magnified by forcing diuresis with the administration of too much water. These patients are best treated by administering fluids to equal output for two to three days and then gradually cutting back on fluid intake. If the kidney responds appropriately, then fluids can be given as the patient requests.