Purpose This study aimed to investigate the bladder capacity (BC) and bladder dynamics of adolescents with anorexia nervosa (AN). Methods The participants consisted of 15 adolescents newly diagnosed with AN according to the DSM 5 criteria and in the acute weight loss period who were questioned about the symptoms of lower urinary tract (LUT) dysfunction. Functional bladder capacity (FBC) and voided volume with uroflowmetry were measured for each subject; the larger volume of the two was chosen for the bladder capacity. Uroflowmetry was used to obtain uroflow curves for the participants whose patterns were labeled as pathologic if they were outside the bell-shape. Results Fourteen (93.3%) of the patients exhibited at least one of the LUT dysfunction symptoms (pathologic voiding symptom/urinary incontinence/pathologic uroflow pattern). BC was observed to increase in 86.6% (n = 13) of the patients. Eighty% of the patients (n = 12) showed pathological uroflow patterns. In patients with pathological uroflow patterns, which showed insufficiency of bladder contraction, assistance of abdominal muscles was needed during voiding. Conclusion The novel findings presented in this study are the increase of BC in adolescents with AN, the presence of at least one type of voiding or bladder dysfunction, and the pathology of uroflow patterns of most patients show that the bladder dynamics is affected in AN. The most important contribution of this study to the literature is that impaired bladder dynamics was determined to be a medical complication of AN.