Introduction: Nocturnal enuresis is a common clinical problem affecting 5% to 10% of school-age children. Etiology is not known but a diminished functional bladder capacity it has been proposed as a predisposing factor. There exist only a few studies evaluating it by ultrasound. Objective: To identify if there is a difference of the functional bladder capacity measured by ultrasound between nocturnal enuresis group and healthy children. Patients and Method: A cross-sectional study from February 2014 to May 2015 including two groups, nocturnal enuresis and a control group of 40 patients each, 5 to 15 years old. A single blinded operator measured the functional bladder capacity by ultrasound with an Acuson S2000 Siemens (TM) 3.5 and 5 MHz transducer. Analytics and descriptive statistics were performed using IBM SPSS 20 (TM) software. Results: Patients with enuresis showed a decreased functional bladder capacity vs. controls (171.7 ml vs 225.5 ml; p = 0.025). A history of first-degree relative with enuresis increased the risk of having enuresis OR = 2.81 (95% CI: 1.067.42), a second-degree relative presented OR = 4.0 (95% CI: 1.48-10.78). Functional bladder capacity presented a weak correlation with the bladder capacity estimated by Kaefer's formula. Conclusion: The functional bladder capacity is lower in the patients with nocturnal enuresis when compared to control group. There is little correlation between functional bladder capacity and Kaefer's formula to determine the normal bladder capacity. We reaffirmed that the family history with enuresis strongly increases the risk of developing nocturnal enuresis.