Diabetes mellitus is usually subdivided into type 1 and type 2. Despite precise criteria, distinction between these two types of diabetes can be difficult because of cases with superposition of the two classes. Adults aged 20 to 40 are particularly at risk of presenting an intermediary type of diabetes and thus are subject to misclassification. The distinction between these subtypes is relevant because of the therapeutic decision and the outcome which relies on insulin supply and therefore the evolution to insulin dependence. Thus, it seems important to review a new and more accurate classification of diabetes to offer a more appropriated care to patients.