A 74-year-old man developed Cushing's syndrome and hypokalemia due to ACTH-independent bilateral macronodular adrenocortical hyperplasia (AIMAH) with excessive secretion of mineralocorticoid hormones. Plasma concentrations of weak mineralocorticoids were high. The increase in plasma cortisol did not have a diurnal rhythm, and was not suppressed by a high dose of dexamethasone. Plasma ACTH was undetectable, but plasma cortisol was increased by ACTH administration. The concentrations of mineralocorticoids, especially deoxycorticosterone and corticosterone were increased, and augmented the response to ACTH administration. Plasma renin activity and aldosterone concentrations were rather suppressed. Both adrenal glands, with a total weight of 110g, were enlarged and contained several macronodules. These nodules were composed of hyperplasia of small cortical cells and usual clear cells. This is a rare case of ACTH-independent bilateral macronodular adrenocortical hyperplasia because there were excessive secretion of mineralocorticoid and hypokalemia.