Pheochromocytomas have been shown to impair glucose tolerance and, rarely, to precipitate overt diabetes mellitus. We report here a case of a large pheochromocytoma in a woman with a recent diagnosis of diabetes mellitus that proved difficult to control despite high-dose insulin therapy who had complete resolution of her hyperglycemia following adrenalectomy. Her dramatic presentation demonstrates the need to consider this etiology in patients with new-onset insulin resistance and hypertension. Keywords: Pheochromocytoma, Insulin resistance, Impaired glucose tolerance, Diabetes mellitus