INTRODUCTION: Primary hyperaldosteronism is only rarely caused by unilateral adrenal hyperplasia.CASE PRESENTATION: A 73-year-old hypertensive Greek man (on 10 mg amlodipine for the last ten years) presented in the emergency department with severe muscle weakness of all limbs. The initial physical and laboratory examination revealed normal blood pressure, muscle weakness, severe hypokalemia, sinus rhythm and U wave, rhabdomyolysis and metabolic alkalosis. The patient was immediately treated with intravenous administration of potassium-rich solutions, 25 mg spironolactone with progressive dose titration up to 100 mg. Because of high arterial blood pressure, irbesartan was added. On day 6, muscle weakness was completely restored with decrease of arterial blood pressure and further improvement of laboratory tests. The combination of hypokalemia with arterial hypertension raised the suspicion of primary hyperaldosteronism; therefore, we performed abdomen computed tomography scan, which revealed a nodular mass (15 mm in diameter) in the left adrenal gland. Plasma renin activity was in the lower normal range with a three-fold increase of plasma aldosterone concentration. We performed total resection of the left adrenal gland and the histopathological examination revealed hyperplasia of the left adrenal gland.CONCLUSION: This report presents a rare case of an elderly patient under antihypertensive treatment the last ten years for essential hypertension, who admitted to our emergency department with hypokalemia - induced myopathy as first manifestation of primary hyperaldosteronism due to unilateral adrenal hyperplasia.