A 43-year-old female was admitted to our hospital for high blood pressure control. She showed hypokalemia with increased urinary potassium excretion, high plasma aldosterone concentration (33.9ng/dl, normal range; 2-12ng/dl) and reduced plasma renin activity (trace, normal range; 0.83-5ng/ml/hr). Bilateral adrenal tumors were revealed by abdominal computed tomography, and left adrenalectomy and right partial adrenalectomy were performed. Cytochromes p-450 and other enzymes involved in aldosterone synthesis were found predominantly in the tumor portions but not in the zona glomerulosa of the attached adrenals, which histopathologically showed paradoxical hyperplasia. This was a rare case of bilateral aldosterone-producing adrenal adenomas, which we could differentiate from idiopathic hyperaldosteronism by employing immunohistochemical analysis of steroidogenic enzymes.