Primary aldosteronism is a syndrome characterized by hypokalemic alkalosis and hypertension. Aldosterone-producing adenomas and bilateral adrenal hyperplasia are common causes of this syndrome. An aldosterone-producing adrenocortical carcinoma is a very rare cause of primary aldosteronism. Recently we experienced a case of an aldosterone-producing adrenocortical carcinoma. A 41-year-old female was admitted for evaluation of a retroperitoneal mass. Because of hypokalemia and a history of hypertension, we evaluated the patient for primary aldosteronism. The high ratio of plasma aldosterone to renin activity suggested the possibility of the presence of primary aldosteronism. We performed adrenal vein sampling for differential diagnosis of an aldosterone-producing tumor from a retroperitoneal mass. The adrenal vein sampling showed that the primary aldosteronism was due to an aldosterone-producing tumor from the left adrenal gland. Surgical findings indicated that the retroperitoneal mass originated from the left adrenal gland and the pathological diagnosis for the mass was an adrenocortical carcinoma. In conclusion, the results from the adrenal vein sampling, as well as the surgical and pathological findings demonstrate that this case was an aldosterone-producing adrenocortical carcinoma.