Chromosomal abnormalities such as Klinefelter syndrome, Down syndrome, Turner syndrome, Prader-Willi, Bardet-Biedl syndrome were associated with diabetes mellitus. Over 30 cases of interstitial deletions of the long arm of chromosome 6 with vastly variable breakpoints and clinical features have been reported in the literature, The clinical findings varies and most often includes mental retardation, microcephaly, and craniofacial anomalies. We report a case of interstitial deletion (del(6)(q21q23)) with type 2 diabetes mellitus and mental retardation (J Kor Diabetes Asso 281~284, 2000).