During insulin hypoglycaemia (IH) reference values for plasma ACTH are 10 to 88 pg/ml at 0 min and 50 to 300 pg/ml at 40 min. Plasma cortisol reference values are above 400 nmol/l at 40 min. A negative correlation between age and ACTH response was found in normal children. ACTH response during IH in 43 children and adolescents with deficient production of one or several pituitary hormones was significantly lower (median 81 pg/ml at 40 min), than the response in normal children (median 149 pg/ml). Thirty-six patients with idiopathic hypopituitarism could be grouped into 21 with normal, 6 with borderline (50 to 70 pg/ml at 40 min), and 9 with abnormal ACTH response. In 7 patients with intracranial tumour 2 had normal, 3 borderline, and 2 abnormal ACTH response. A good correlation between the ACTH peak at 40 min and the cortisol value at 40 min was found in patients with idiopathic hypopituitarism. ACTH response also correlated well with the plasma growth hormone (GH) response in the patients. It is recommended to study ACTH together with GH when IH is performed, multiple disturbances are often found in children with GH deficiency and demonstration of an ACTH defect has therapeutic implications.