A unique syndrome in a three-year-old American Indian girl was characterized by signs and symptoms of mineralocorticoid excess in the absence of excessive secretion of any known sodium-retaining steroids. Hypertension and hypokalemic alkalosis were corrected by spironolactone or a low sodium diet. Plasma renin activity was suppressed but the secretion of aldosterone was undetectable and was not stimulated by salt depletion. There was no evidence of abnormal accumulation of aldosterone precursors and metabolism of a tracer dose of the hormone was normal. Secretion rates of cortisol, corticosterone, deoxycorticosterone, deoxycortisol and aldosterone were very low and did not increase normally with ACTH administration. However ACTH administration aggravated hypertension and hypokalemia. Dexamethasone did not improve hypertension. Despite low secretion of glucocorticoids and mineralocorticoids, the patient showed no addisonian features and survived severe illness. Secretion of a factor of adrenocortical origin was suggested by the exacerbation of the syndrome of ACTH. The unidentified factor appears to be both a potent glucocorticoid and mineralocorticoid.