Aldosterone-producing adenoma (APA), one of the most common causes of primary hyperaldosteronism, is clinically characterized by hypertension, increased sodium retention, increased potassium excretion and altered glucose metabolism. APA can also manifest in the form of hyperosmolar hyperglycaemic state (HHS), which is a life-threatening acute diabetic complication. Infection and inadequate fluid replacement are the common precipitating and predisposing factors for the occurrence of HHS. Here, we report a case of occult APA with the initial presentation of HHS, in which an unusually rapid correction of diabetes was observed after correct diagnosis and surgical resection of the APA. To our knowledge, this is the first case with such manifestations and outcome.