Bladder capacity, nocturnal urinary volume and nocturnal urinary behavior were investigated in 1751 nonenuretic and 282 enuretic children in two primary schools and four kindergartens. The results were as follows: 1. A statistically significant linear correlation was observed between the age and the bladder capacity in the morning and daytime, as well as nocturnal urinary output, in nonenuretic children. 2. The bladder capacity of enuretic children was smaller in the ages below 6 but larger in the ages over 7 than that of noneuretic children. It was not likely from this evidence that an immature bladder capacity was a primary cause of enuresis. 3. For the estimation of the maximum bladder capacity in the study of enuresis, urinary output in the morning immediately after awakening with sufficient urinary sensation was thought to be more reasonable than that in the daytime, because output in the morning was approximately 50% more than that of the daytime. 4. Ten to 15% of nonenuretic children showed nocturnal polyuria with nocturnal urination after complete awakening more than one time every week. It was also not likely from this evidence that polyuria was a primary cause of enuresis. 5. The overall occurrence rate of enuresis was 14%, higher in males up to the ages of 9, and almost equal in males and females after the ages of 10. 6. The average age for the spontaneous disappearance of enuresis was 7.3 y.o., therefore, adequate treatments for enuresis should be instituted after the age of 8.