A 29-year-old woman with deoxycorticosterone (DOC)-producing adrenocortical adenoma had hypertension and hypokalemia but without Cushingoid features. Plasma renin activity and the aldosterone concentration were low, while the DOC concentration was high (6.10-10.3 ng/ml; normal range 0.03-0.33). Plasma cortisol, androgens, and estrogens as well as urinary 17-OHCS and 17-KS were within normal limits. Furosemide administration and two hours upright posture resulted in a 3-fold increase in plasma DOC, but the administration of ACTH, dexamethasone, or angiotensin III had no effect on plasma DOC. Following resection of a right adrenal tumor weighing 70 g, the hypertension and hypokalemia disappeared. DOC content in the tumor was high. On light microscopic examination, the tumor was encapsulated, composed of cells with clear cytoplasm and large nuclei and there were extensive areas of fibrosis and infiltration of lymphocytes. According to Weiss's criteria, the tumor was considered to be an adrenocortical adenoma. Immunohistochemically, P450scc, 3 beta HSD, P450C21 and P45011 beta were positive with heterogeneity of intra-tumoral expression. No immunoreactivity for P45017 alpha in this adenoma was detected. This is different from a previous report in which a relatively small number of cells in DOC-secreting adrenocortical carcinoma were positive for P45017 alpha.