Objectives. An increase in volume and evidence of postvoiding residuals are both encountered in the bladder of diabetic patients and can lead to urinary tract infections and impair renal function. Our objective was to compare the bladder volume of diabetic children and adolescents with that of nondiabetic subjects. Methods. We investigated 247 diabetic patients and 228 control subjects between the ages 3 and 21 years. The sonographic examination was performed with a full bladder, after spontaneous voiding, and after forced voiding. Three dimensions were determined: longitudinal, transverse, and anteroposterior transverse. The data were analyzed by the χ 2 test, analysis of variance, the Kruskal-Wallis test, and multivariate regression. Results. The 2 groups were equivalent for age (P = .56) and sex (P = .82). The median full bladder volume was larger in the diabetic group (268 mL) than in the control group (220 mL; P = .0004). Postvoiding residual volume after spontaneous and forced voiding was significantly higher in the diabetic group (P < .0001). Multivariate analysis showed that being diabetic (P = .021), older than 9 years (P = .000), and female (P = .036) all influenced full bladder volume. Conclusions. Sonographic evaluation showed incipient bladder dysfunction in diabetic patients. The multivariate analysis showed that the increase in bladder volume had a association with age (>9 years), female sex, and mean glycosylated hemoglobin value. Thus, bladder volume evaluation by sonography should be incorporated in the routine assessment of patients with type 1 diabetes.