In summary, a natriuresis and diuresis following the relief of BUO is common. This is frequently associated with the reversal of hypertension and other signs of salt and water retention. The need for replacement therapy is best determined from clinical assessment of salt and water status. Only rarely will a prolonged salt-losing state ensue, but the physician must be aware of this possibility, since long-term replacement of water and electrolytes may be required. Recovery of function occurs in two phases, an early tubular phase and a later glomerular phase. The tubular phase appears to be quantitatively more important. There is some disparity between the improvement in creatinine clearance and that in 99mTc DTPA and iohexol clearance, and we would postulate that the initial improvement in creatinine clearance is due to secretion of creatinine during the predominantly tubular phase of recovery. It follows that when assessing renal functional recovery after insertion of a nephrostomy tube or other form of decompression of an obstructed upper urinary tract, these different phases must be borne in mind. Following the relief of UUO, changes in water and electrolyte excretion do occur, but they are rarely of clinical significance.