OBJECTIVE: To study the association of functional bladder capacity with the severity of bedwetting in children with nocturnal enuresis. METHODS: A questionnaire investigation was performed in 1 500 children with nocturnal enuresis and the functional bladder capacity was examined by B-ultrasound. RESULTS: The ratio of males to females was 1.3:1. The majority of patients (87%)were in an age range of 5-10 years, followed by the 10-14 years group (12%), and the 15-18 years group (1%). Six hundred and thirty-seven patients (42.4%) showed a decreased functional bladder capacity (less than 50% of normal level). The patients were classified into four groups according to the severity of bedwetting (from severe to mild): ≥2 times per night (n=53, 3.5%), ≥7 times per week (n=969, 64.6%), 3-6 times per week (n=380, 25.3%) and 1-2 times per week (n=98, 6.5%). The incidence of the reduction in functional bladder capacity in the above four groups was 79.2%, 48.3%, 29.7% and 14.3% respectively and a significant difference was noted among the four groups. CONCLUSIONS: Most of children with nocturnal enuresis showed decreased functional bladder capacity. Functional bladder capacity is associated with the severity of bedwetting in children with nocturnal enuresis.