Neonatal diabetes mellitus (NDM) is characterized by hyperglycemia within the first month of life and insulin dependence for at least two weeks. There are two types of NDM, transient (TNDM) and permanent (PNDM), which are genetically different. We report the case of two brothers who developed hyperglycemia without ketosis on the 18th day and 2 h of life, respectively. Thyroid function tests, abdominal ultrasound and karyotype where normal and there were no pancreatic antibodies. The first one required insulin therapy for the first 92 days of life and the second for 5 months. The mother developed gestational diabetes during both pregnancies and she was later diagnosed diabetes mellitus (without antibodies). They were studied for mutations in KCNJ11 gene (principally related to the permanent form). The three of them showed the E229K mutation (frequently associated with the transient form). A genetic study is essential in NDM to achieve the most accurate prognosis possible.