Common fibular (peroneal) nerve palsy has many etiologies. A rare cause with a poorly understood pathophysiology is common fibular nerve palsy that follows an ankle injury. Since the original description of this pathologic entity a century ago, several mechanisms have been posited. We present two cases of common fibular nerve palsy following ankle injury. These cases provide anatomic and radiologic evidence to support a hypothesis that the interosseous membrane provides the anatomic basis for this combined pattern of injury. This theory unifies previously postulated mechanisms. Clin. Anat. 25:401–406, 2012. © 2011 Wiley Periodicals, Inc.