Abstract We report on a 6-year-old child presenting with subacute foot drop. Neurophysiologic and radiologic studies revealed a peroneal nerve compression secondary to fibular exostosis. Before undergoing surgical removal of the exostosis, the patient underwent further neurophysiologic and ultrasonographic evaluation that showed the presence of an accessory peroneal nerve branch that caused gastrocnemius involvement. Findings at surgery confirmed the supposed anatomical variant. Both nerve components were carefully preserved during the operative procedure. The association of ultrasonographic and neurophysiologic studies was crucial in identifying the etiopathologic mechanism and anatomical picture and provided clinicians and surgeons with important information in planning the procedure.