A 46-year-old woman was presented with mineralocorticoid excess syndrome and a large mass originating from the right adrenal gland. Clinical examination before right adrenalectomy revealed elevated serum concentrations of 18-hydroxy-11-deoxycorticosterone (18-OH-DOC) both systemically and in the adrenal veins bilaterally. Histopathological and immunohistochemical analyses of the surgical specimen demonstrated adrenal hyperplasia of outer fasciculata cells, and the presence of cystic mass. The adrenalectomy ameliorated her blood pressure (BP) from 156/96 mmHg to 148/87 mmHg with a concomitant increase of serum potassium concentration from 3.1 mEq/l to 3.5 mEq/l. These results suggest that uni-adrenalectomy is, at least in part, effective in ameliorating not only BP but also potassium concentration in a patient of adrenal hyperplasia with excessive bilateral 18-OH-DOC production.