Activating mutations in the KCNJ11 gene encoding the ATP-sensitive potassium-channel subunit of Kir6.2 result in the phenotype of permanent neonatal diabetes (PNDM). Patients with PNDM can be successfully transferred from insulin to sulphonylurea. It is not clear, however, whether the type of diet may play a role in the metabolic control in PNDM patients. This report describes two cases of patients with PNDM due to the R201H mutation coming from the Polish Nationwide Registry of PNDM treated with the same sulphonylurea (glipizide GITS). In one of them, diet was practically free (Pol1), the other one (Pol2) avoided high glycemic-index products. Both mutation carriers were submitted to a 72 h continuous glucose monitoring system (CGMS) (Medtronic, CA). Before the CGMS record, families were encouraged not to alter their usual pattern of food intake during recording periods and to use food diaries. The postprandial glycemia in Poll reached the maximal level of 9.5 mmo/l, 5 episodes of glycemia above 8.0 mmol/l lasting overall for about 6 hours followed the ingestion of high-glycemic-index (>70) meals. Patient Pol2 did not use high-glycaemic-index-products and his postprandial blood glucose did not exceed 7.0 mmol/l. Following the CGMS record, an additional diet-oriented educational session with patient Poll and his parents was performed, Poll declared to avoid the intake of high-glycemic-index products. He remained on the same dose of Glipizide GITS. Results of home blood glucose monitoring performed 2 months later showed normoglycemia. We conclude that to achieve normoglycemia, patients with PNDM who are on sulphonylurea should refrain from eating high glycemic-index products.