Although radial nerve palsy associated with a closed humeral shaft fracture may be managed by observation, it is our experience that an open humeral shaft fracture with radial nerve palsy requires exploration of the nerve. In a series of 14 patients with radial nerve palsy caused by an open humeral shaft fracture, 9 (64%) of the 14 patients had a radial nerve that was either lacerated or interposed between the fracture fragments. There was an equal incidence of radial nerve lacerations or entrapments in types I, II, and III open humeral shaft fractures. Epineural radial nerve repair, done primarily or secondarily, provided a satisfactory return of radial nerve function. Rigid fixation of the associated fracture is the recommended treatment.