Prader-Willi(PW)syndrome is characterized by obesity, hypotonia, mental retardation, hypogonadism, short stature, excessive eating and characteristic facial appearance. Diabetes mellitus has been considered a component of PW syndrome. Recently this syndrome is caused by the absence of paternally derived genes normally located on chromosome segment 15 q11-q13 or may be the result of maternal uniparental disomy with the absence of paternally derived 15 q11-q13 region. The developement of probes containing segments of DNA from chromosome region 15 q11-q13 provides the oppotunity to confirm the diagnosis of PW syndrome by fluorescence in situ hybridization(FISH). We experienced a 15-year-old boy of PW syndrome with diabetes mellitus, who revealed mental retardation, hypogonadism, obesity and microdeletion of chromosome 15 q11-q13 comfirmed by FISH.