Ureteral obstruction can have a variety of causes intrinsic or extrinsic to the kidney. The effects of obstruction are examined from the perspectives of duration, severity, totality, and the presence of complicating factors. There is a difference in the postobstructive pathophysiology depending on whether one or both ureters were obstructed. Atrial natriuretic peptide may be important in postobstructive diuresis, and preliminary evidence suggests a role for it as protection against nephron ischemia in acute obstruction. The potential for recovery of renal function after relief of obstruction depends on the duration and degree of obstruction, the condition of the contralateral kidney, and the presence or absence of infection. Ability to acidify the urine to pH < 6.0 preoperatively may be a good predictor of the recovery potential of an obstructed kidney. Urine concentrations of lysosomal enzymes such as N-acetylglucosaminidase also may be useful for this purpose, as may measurement of creatinine clearance in urine obtained from a nephrostomy tube.