Lethal sphenoid fungal sinusitis in an immunocompromised elderly patient. Sphenoid fungal balls are rare and typically indolent lesions, unless superinfected by bacteria or invasive to adjacent neurovascular structures. If the identification or treatment of underlying complications is delayed in an immunocompromised patient, a catastrophic outcome may result. We report the case of an elderly female patient with poorly controlled diabetes mellitus suffering from sphenoid fungal sinusitis that is complicated by cavernous sinus thrombophlebitis and carotid artery stenosis. In spite of reasonable diagnosis and therapy, the patient’s general condition deteriorated and she eventually died. The clinical presentation, diagnosis, and treatment strategy are discussed.