Permanent neonatal diabetes mellitus (PNDM) is a rare form of diabetes diagnosed within the first 6 months of life. Heterozygous activation mutations in KCNJ11, encoding the Kir6.2 subunit of the ATP-sensitive potassium (K(ATP)) channel, which acts as a key role in insulin secretion regulation, account for about half of the cases of PNDM. The majority of the patients represent isolated cases resulting from de novo mutations. Approximately 20% have associated neurologic features: the most severe form, which includes epilepsy and developmental delay, is called developmental delay, epilepsy, and neonatal diabetes (DEND) syndrome and the milder form, with less severe developmental delay and without epilepsy, is designated intermediate DEND syndrome. Individuals with KCNJ11 mutations have been successfully transitioned from insulin to sulfonylurea (SU) therapy. Furthermore, there have been cases reported with variable improvement in neurological function following a successful switching. We describe a 12-year-old Portuguese girl with PNDM due to the previously reported R201C mutation in the KCNJ11 gene. Her medical history includes prematurity and moderate developmental delay. The mutation was inherited from her mother who has isolated PNDM. The patient was successfully transferred from insulin to SU, whereas her mother showed SU resistance. Despite good glycemic control, no improvements in the cognitive performance were verified. We present our experience in switching treatment from insulin to oral SUs in this family, and also discuss whether or not the girl's developmental delay is related with the Kir6.2 mutation. To our knowledge, this is the first Portuguese patient reported with successful transition to SU treatment.