Smith-Magenis syndrome (SMS) is a syndrome of multiple malformations caused by an interstitial deletion of chromosome 17p11.2. We report the case of an 8-year-old boy with SMS. Down syndrome was initially suspected in infancy based on the findings of generalized hypotonia, flat midface, and upslanting palpebral fissures. His sleep had been disturbed since infancy, and self-injurious behavior developed at 2 years. When he was 8 years old, these unusual neurobehavioral features led to suspicion of SMS, and chromosome analysis showed the 17p deletion, which was confirmed by fluorescence in situ hybridization of the SMS region. Bilateral vesicoureteral reflux, grade IV, was found at the same time, and he underwent bilateral ureteroplasty. The postoperative course was smooth and he was discharged with antibiotic prophylaxis. His sleep disturbance improved after treatment with melatonin. A high index of suspicion is needed for the timely diagnosis of SMS. Patients should be thoroughly evaluated for associated complications both at the time of diagnosis and at regular follow-up.