Early morning urine osmolality in 1,453 nonenuretic children and 144 enuretic children was measured in a population study. Early morning urine osmolality was lower in nonenuretic children aged 3-4 years than in those aged over 5 years. A similar pattern was observed in enuretic children. No significant difference in early morning urine osmolality was observed between nonenuretic children and enuretic children at any age. These data suggest that nocturnal urine concentration may be immature in nonenuretic children under 4 years. We did not find any evidence for increased nocturnal urine production as a cause of enuresis.