We report clinical findings and steroidogenic activities in adrenal tissues in 2 cases of AIMAH. Endocrine studies revealed an undetectable level of plasma ACTH and a diminished circadian rhythm of plasma cortisol. A significant increase in plasma cortisol levels in response to ACTH stimulation was observed in both cases. After the administration of metyrapone in one case, urinary excretion of 17- hydroxycorticosteroid (17-OHCS) significantly increased, although the plasma ACTH level did not respond. Computed tomography showed large masses in both adrenal glands, and bilateral uptake was identified on adrenal scintigraphy. The totals for the bilateral adrenal glands were 98g and 105g, respectively, and the left adrenal was larger than the right in both cases. Steroid content in the nodules easured by high performance liquid chromatography (HPLC) showed that the cortisol content was definitely lower than that in cortisol-producing adenoma (CPA) and even in normal adrenals. The activities of cytochrome P450c17, P450c21 and P450c11 were evaluated in one case, and all of them were reduced in the nodules. Especially that of P450c17 was remarkably reduced. These data suggest that cortisol production in AIMAH is inefficient, and that the cause of Cushing's syndrome may be related to the marked increase in the number of cells or bulk of the tumor.