Renal dysplasia and atrophy are associated with congenital urinary tract obstruction, such as posterior urethral valves, and are thought to result from the increased pressure in the developing obstructed urinary system. This increased pressure is likely to be alleviated by perirenal urinary extravasation and urinary ascites. Whether this relief of pressure in the obstructed developing kidney results in a lessened degree of renal abnormality is likely to depend on the time during the period of gestation when the urinary leak occurs and the degree of renal dysplasia and atrophy at the time of the urinary leak.