Rett syndrome (RS) is a neurodevelopmental disorder mainly affecting girls. It is characterized by a normal prenatal and perinatal period, apparently normal development for the first 6 months of life, and then a decelaration in head growth, loss of hand and communication skills, psycho motor retardation, as well as the development of sterotyped hand movement and truncal or gait apraxia. It has been shown to be related to mutations in the MECP2 gene located on Xq28. Diabetes mellitus (DM) type 1 may be associated with certain genetic disorders such as Down syndrome, Turner syndrome, and Klinefelter syndrome. In this work, we report the case of a 9-year-old girl with RS who developed DM at the age of 6. To our knowledge, our patient is the third case reported to date of DM associated with Rett syndrome. © 2012 by Walter de Gruyter.