Smith-Magenis syndrome (SMS) is a multiple congenital anomalies/mental retardation syndrome characterized by an interstitial deletion on the short arm of chromosome 17 involving the band p11.2. A 3-year-old girl was referred for evaluation of moderate psychomotor retardation and several behavioral problems including self-injuring behavior, hyperactivity, and sleep disturbance. Visual and hearing impairment, and brain abnormalities including ventriculomegaly and hypoplastic right transverse sinus, were detected. Routine cytogenetic study showed an apparent mosaicism of an interstitial deletion over the chromosomal region 17p11.2 in 65% metaphases. However, the microdeletion was found in all metaphases by fluorescence in situ hybridization study with the probe D17S29 (for Smith-Magenis critical region on 17p11.2). Her sleep disorder improved after the use of melatonin.