A 46-year-old male with long-term treatment-resistant hypertension and past history of cerebral hemorrhage was found to have suppressed plasma renin activity (PRA) and normal plasma aldosterone concentration (PAC) with aldosterone/renin ratio of 25.3. Furosemide plus upright test did not stimulate PRA, but computed tomography scan of the abdomen revealed no abnormal lesions in either adrenal gland. Selective adrenal venous sampling (SAVS) showed that PAC in the left and the right adrenal vein were 1000 ng/dl and 230 ng/dl, respectively, which increased to 1500 ng/dl and 620 ng/dl, respectively, after ACTH stimulation. Diagnosis of primary aldosteronism due to hypersecretion of aldosterone from the left adrenal gland was made, and laparoscopic left adrenalectomy was performed. Pathological examination of the 'apparently normal' adrenal tissue resected revealed the presence of poorly encapsulated multiple adrenocortical micronodules which showed positive immunoreactivity for 3beta-hydroxysteroid dehydrogenase by immunohistochemical study, but negative immunoreactivity in the hyperplastic zona glomerulosa consistent with paradoxical hyperplasia associated with primary aldosteronism. Postoperatively, PRA was normalized and his high blood pressure was well controlled with lower doses of antihypertensive drugs than those used before surgery. The clinicopathological features of our case are consistent with the diagnosis of unilateral multiple adrenocortical micronodules (UMN), a new subset of primary aldosteronism, in which SAVS proved to be a useful diagnostic tool for its localization.