A total of 3 neonates exhibited severe post-obstructive diuresis after relief of unilateral ureteral obstruction. Of the neonates 2 had normal kidneys contralaterally while 1 had a contralateral kidney that was dysplastic. Urinary output exceeded oral intake initially in all 3 patients necessitating intravenous supplementation for up to 10 days. Stabilization resulted from an increase in oral intake rather than a decrease in diuresis. Unilateral post-obstructive diuresis is extremely uncommon but it may occur in the neonate because of the unique features of the renal physiology encountered in this age patient.