Maturity Onset Diabetes of the Young (MODY) is a monogenic form of diabetes with onset in patients aged less than 25 years. It is a heterogeneous disorder due to heterozygous monogenic mutations with an autosomal dominant transmission. It could represent 2 to 5% of diabetes but is often under-diagnosed. We report three different cases of MODY, two without associated abnormalities and one with renal disorder. Mutations concern genes that are directly involved in the beta-cell function. In patients with non-syndromic diabetes, more than 99% of MODY result from mutations in hepatocyte nuclear factor-1-alpha (HNF-1-alpha ; formerly MODY 3), glucokinase (MODY 2), or HNF-4-alpha (MODY 1). The symptoms manifest slowly with the absence of obesity and ketosis in most cases. MODY is usually treated by diet, oral diabetes medications or insulin. Treatment and prognosis vary depending on the genetic mutation. Clinicians should keep in mind the possibility of MODY, especially in antibody-negative youth with familial diabetes. Making a diagnosis of MODY may have important implications for the guidance of appropriate treatment, prognosis and genetic counselling.