The classification of diabetes mellitus is not always straightforward but is of clinical importance in view of the differences in treatment, prognosis and inheritance. In young patients with acute signs ofhyperglycaemia and a low bodymass index (BMI), diabetes type 1 is the evident diagnosis, and insulin therapy should be initiated as quickly as possible. Deafness in patients with mild symptoms and a highly positive family history suggests the presence of a mitochondrial or maternally inherited type of diabetes: maternally inherited diabetes and deafness (MIDD). In the absence of deafness, maturity onset diabetes of the young (MODY) should be suspected in the same group of patients. In case of a non-acute presentation of hyperglycaemia without an evident family history of diabetes, the BMI can be helpful in the next steps of the diagnostic process. In practice, the proposed flow chart can readily be applied for the classification and treatment of patients with hyperglycaemia.