Nesidioblastosis is a rare metabolic disease characterised by inappropriate insulin secretion often associated with life-threatening hypoglycaemia. While severe cases present in the newborn period, patients have been described later in infancy. Familial cases suggest an autosomal recessive trait, and recently mutations in the sulphonlurea receptor gene, possibly a regulator of insulin secretion, have been identified and associated with disease expression. We report a twin boy who developed normally until the age of six months when he was noted to regress. The boy is the older twin born to non-consanguinous parents. He presented to a hospital first at the age of 13 months with fever and generalised seizures. Low blood glucose was noted, but he recovered easily and was able to maintain euglycaemia during a 48-hour period of observation. Microcephaly and developmental delay were documented and anticonvulsant therapy was started. At 18 months, low blood glucose with high C-peptide was documented during reevaluation. Follow-ing a short trial of subcutaneous long-acting somatostatin analogue, the child was subjected to near-total pancreatectomy. The histology revealed findings consistent with nesidioblastosis. The child's condition improved but he remained significantly delayed This case emphasises the importance of recognising and treating hypoglycaemia early to avoid irreversible brain damage. It is interesting to note that the twin brother has always been well and is developmentally normal. Further studies to identify the inheritance pattern in the family would be of great interest.