Abstract
A right flank mass, in a patient with fever of unknown origin, pain, and superiorly displaced right kidney on excretory urogram, was explored through a subcostal incision. Finding of a retroperitoneal abscess was anticipated; instead a ruptured mycotic aortic aneurysm was encountered. An awareness that entities such as this may exist is imperative in the differential diagnosis of flank masses. Treatment through a flank incision presents an elusive if not impossible feat. Because of the gravity of the underlying disease, misdiagnosis almost always results in death.