A two-month-old Danish girl was admitted to the hospital in diabetic ketoacidosis and diagnosed with permanent neonatal diabetes mellitus (PNDM). She received continuous insulin treatment until she was genetically tested at the Steno Diabetes Centre. She carried a KCNJ11 Arg201His mutation, an activating mutation in the KCNJ11-gene which encodes the ATP-sensitive potassium subunit Kir6.2 in the beta cell which is responsible for insulin secretion. As recommended in the literature, she was successfully shifted from insulin therapy to sulfonylurea tablets at the age of three years and nine months. PNDM-patients should be screened for gene mutations regardless of current age.