Seventy cases of ureteropelvic junction obstruction, bilateral or unilateral, were followed prospectively throughout gestation and postnatally for an average of 2.3 years. Cases of ureteropelvic Junction obstruction with a renal pelvis dilated less than 1 cm uniformly did well; those with a pyelectasis more than 2 cm, both bilateral and unilateral, had a favorable outcome in approximately three-quarters. Surprisingly, pelvis dilatation between 1-2 cm had a better outcome if bilateral than if unilateral.