Bilateral adrenal aldosterone-producing adenomas (APA) are rare. It is important to distinguish bilateral APA from idiopathic hyperaldosteronism (IHA) which is due to bilateral hyperplasia of the adrenal cortex. We present two patients with bilateral APA in whom the diagnosis was made histochemically by analyzing steroidogenic enzymes. They showed hypokalemia, high plasma aldosterone concentration (PAC) and suppressed plasma renin activity (PRA). Bilateral adrenal tumors were represented by computed tomography, and surgical resection was performed. In both cases, cytochrome P-450 and other enzymes that were involved in aldosterone synthesis were found mainly in tumor, but little in the zona glomerulosa of the adjacent adrenals, which showed paradoxical hyperplasia. Such cases are difficult to distinguish from IHA. The two disorders were differentiated by immunohistochemical analysis of steroidogenic enzymes.