The association between maturity onset diabetes of the young (MODY) and type 1 diabetes mellitus (T1DM) has been rarely described. We report two patients affected by MODY who developed T1DM. Case 1: a 4-yr-old girl referred for glycosuria presented hemoglobin A1c (HbA1c) of 6.6%. Islet cell antibodies (ICA) and anti-glutamic acid decarboxylase (GADA) were initially negative. As her father, uncle and grandmother showed mild hyperglycemia, they were screened for MODY 2. A novel mutation in glucokinase gene was found in the family. Few months later, her glycemic control worsened consistently and she required insulin treatment. A high titer of GADA and ICA was then detected. Six years afterwards insulin requirement is 0.8 U/kg and HbA1c 6.7%. Case 2: a 15-yr-old boy treated for growth hormone deficiency was found with a blood glucose level of 106 mg/dL. HbA1c was 7.2%, ICA and GADA were negative. Family history was positive for autoimmune diseases and type 2 diabetes mellitus. The patient was investigated for MODY 2 and MODY 3, and a mutation of hepatocyte nuclear factor-1 alpha gene was found. The same mutation was found in the mother who had never been referred for hyperglycemia. After 1 yr, due to an unjustified worsening of the metabolic control, autoimmunity was again investigated and a mild positivity was found. He then required insulin therapy and after 5 yr current HbA1c was 8.2%. The diagnosis of MODY does not exclude the risk of developing T1DM. Therefore autoimmunity should be investigated when ordinary treatments fail and metabolic control unexpectedly worsens.