Rationale: Primary hyperaldosteronism (PA) is the most frequent and possibly curable form of secondary hypertension. The diagnosis and targeted treatment of PA is essential because of high vascular morbidity associated with PA as compared to essential hypertension with comparable blood pressure levels. PA is usually caused by either a unilateral aldosterone-producing adenoma (APA) or by bilateral adrenal hyperplasia (BAH). Distinction between APA and BAH is critical since the former may be cured by adrenalectomy, and the latter needs life-long medical therapy with mineralocorticoid receptor antagonists (MRA). Studies demonstrate that adrenalectomy benefits also BAH patients with dominant nodule(s) producing the most of aldosterone excess. The distinction between unilateral and bilateral PA can be made by adrenal vein sampling (AVS), as recommended by The Endocrine Society 2008 guideline. Currently, in Finland the diagnosis is based on computed tomography (CT) scanning which does not distinguish between aldosterone-producing and common non-functioning adrenal nodules and has limited accuracy detecting small adrenal masses. Since AVS is invasive, dependent on skilled radiologist and costly, there is a need for an accurate, non-invasive functional imaging such as 11C-metomidate positron emission tomography (MTO-PET).