A 12-year-old girl with superficial moniliasis of a fingernail had idiopathic hypoparathyroidism. Nine months later, she had a clinical syndrome consistent with Addison disease. However, the serum cortisol and urinary 17-hydroxycorticosteroid values were normal. Stimulation with corticotropin and metyrapone showed an intact pituitaryadrenal axis. Plasma renin level was elevated. Despite severe hyponatremia, excessive urinary sodium loss, and evidence of hypovolemia, the excretion of aldosterone and of the major metabolite of 18-hydroxycorticosterone was markedly decreased. However, the excretory product of 18-hydroxydeoxycorticosterone, produced in the zona fasciculata, was normal. These findings point to either an isolated 18-hydroxylase deficiency limited to the zona glomerulosa or to destruction of the adrenal cortex involving only the zona glomerulosa.