A 69-year-old woman, who had been undergoing treatment for hypertension, was admitted to the hospital for the evaluation of a tumor in the right adrenal gland and of episodic hypertension. She was diagnosed with pheochromocytoma on the basis of elevated urine catecholamine and metabolite concentrations and positron emission tomography (PET) results; however, the results of 131I-metaiodobenzylguanidine (131I-MIBG) scintigraphy were negative. The patient underwent laparoscopic right adrenalectomy. Malignancy was suspected on the basis of histopathologic studies. After surgery, the elevated catecholamine and metabolite concentrations and the blood pressure returned to normal; furthermore, the symptoms, including severe headache, reduced. The authors report a case of pheochromocytoma that was suspected to be malignant on the basis of histopathologic analysis and was detected by 18F-fluorodeoxyglucose (FDG)-PET but not by MIBG. Thus, FDG-PET may be a prognostic marker for malignant pheochromocytomas with or without metastasis.