The frequency of hypoglycemia in 165 children with primary adrenal insufficiency, 118 of whom had Congenital Adrenal Hyperplasia and 47 Addison's Disease, was 18%. Half of the hypoglycemic episodes occurred in the neonatal period. Hypoglycemia was isolated in 13 children, revealing the disease in 4 newborns with Congenital Adrenal Hypoplasia and in a boy with 11 B Hydroxylase deficiency. Basal plasma cortisol levels were significantly lower in those of subjects who experienced hypoglycemia (47.1 +/- 28.6 ng/ml vs. 106.0 +/- 86.6 ng/ml, p less than 0.001). A significant correlation (p less than 0.001) was found between the plasma concentration of glucose and cortisol at time of hypoglycemia.