Hyperinsulinaemic hypoglycaemia in infancy may be transient or persistent. Persistent hypoglycaemia is caused by nesidioblastosis of the pancreas, a diffuse pancreatic endocrine abnormality involving all the endocrine tissue, or by an islet cell adenoma. The cause of nesidioblastosis is not known but unless controlled its hypoglycaemic effects lead to convulsions, mental retardation or death. Patients with transient hyperinsulinaemic hypoglycaemia, and some with persistent hypoglycaemia, respond to medical therapy but those with an adenoma and many of those with nesidioblastosis will require partial or total pancreatectomy. The surgical treatment of six infants and a child is discussed. Five infants had nesidioblastosis; one infant and one nine-year-old girl had an islet cell adenoma. It is suggested that in the very young when there is no evidence of an insulinoma, resection at the initial operation should be greater--up to 90 per cent--than the previously recommended 75 per cent.