Although nocturnal enuresis was first described centuries ago, there is still a lot unknown about its pathophysiology. The functional bladder capacities, diurnal vasopressin levels, urine osmolalities and urine output of enuretic and normal children were compared. We have concluded that enuretics have normal bladder capacities insufficient for increased nocturnal urine volumes because of loss of diurnal variation in serum vasopressin levels and related decrease in urinary osmolalities.