Metabolic and endocrine studies on a 7-year-old boy who presented with hypoglycaemic convulsions are reported in detail, proving the diagnosis of isolated ACTH deficiency--a rare cause of hypoglycaemia in childhood. Adrenaline secretion during insulin-induced hypoglycaemia was reduced. Low blood alanine levels occurred during starvation-induced hypoglycaemia, together with raised total blood ketone bodies; blood glucose did not increase adequately after oral alanine at this time. Hypoglycaemia in isolated ACTH deficiency appears to be due to a combination of impaired alanine mobilisation and a decreased rate of gluconeogenesis.