A newborn infant with severe hypoglycaemia and nesidioblastosis was subjected to subtotal pancreatectomy without any sign of improvement. In spite of very low plasma levels of glucose (i.e. less than 1 mmol/l) plasma insulin concentrations were high (i.e. greater than 700 pmol/l). Plasma proinsulin was considerably enhanced comprising 43% of the total insulin immunoreactivity. Plasma glucagon concentrations were normal. Postoperatively normal to subnormal plasma glucose levels could only be maintained by treatment with frequent meals, diazoxide and intramuscular injections of a long-acting glucagon preparation. With time, signs of mental retardation became obvious.