We describe a girl with triple X syndrome and paternal isodisomy of chromosome 6 (UPD6), who developed neonatal diabetes mellitus (NDM) and precocious puberty. At birth she presented growth retardation and congenital anomalies (ventricular septal defect, macroglossia, umbilical hernia). Diabetes mellitus (DM) was diagnosed at 31 days of life and treated with insulin for 13 months. DM recurred at 4 years of age and since that time it required insulin, in spite of preserved beta-cell function. Tall stature was present from early childhood. At 7 years of age the girl presented central precocious puberty, height velocity further increased, but her near-final height was normal. This patient is unique in that precocious puberty has never been described in triple X females. Moreover it is a further example of paternal UPD6 causing NDM with a predisposition to type 2 DM in later life.