In Duchenne muscular dystrophy (DMD), sphincter muscles tend to be clinically spared. However, urinary incontinence is occasionally reported, usually late in the course of the disease. We wished to determine the etiology of urinary dysfunction in patients with DMD. Seven boys with DMD and urinary dysfunction were examined by a neurologist and a urologist followed by urodynamic and electrophysiological assessment. Based on the results of these evaluations, patients were defined as having an upper motor neuron (UMN), lower motor neuron (LMN), or myopathic lesion. Five of the patients had UMN abnormalities consisting of either uninhibited contractions or bladder/sphincter dyssynergy. One patient had a LMN lesion with prolonged duration and high-amplitude motor units. No patient demonstrated myopathic motor units. Five boys had undergone spinal fusion for scoliosis. We conclude that urinary incontinence in DMD is most often due to UMN dysfunction and not due to a severe myopathy of the detrusor or external sphincter. The most likely causes of the UMN abnormalities are severe scoliosis or a complication of spinal fusion surgery.