Injection drug users frequently present to emergency departments with fever. A careful history and physical examination with attention to anatomic localization of symptoms and signs are often necessary to unmask unusual underlying medical conditions. We report a case of a woman with recent injection drug use who presented with fever, a palpable neck mass, and Pancoast's syndrome. She had been seen recently at the ED of another hospital and discharged with oral antibiotics for presumed cellulitis. A mycotic aneurysm of the subclavian artery causing Pancoast's syndrome was later diagnosed by using computed tomography and angiography. A high index of suspicion for anatomically localized infective processes should always be maintained with febrile injection drug users.