A delay in diagnosis of a traumatic radial-head dislocation in the absence of a concurrent ulnar fracture is common. Published reports of this injury imply an isolated injury to the radius without involvement of the ulna. This hypothesis is challenged by a retrospective study of all cases over an 8-year period that demonstrates an identifiable injury to the ulna in every case. A new radiographic sign, the "ulnar bow sign," is described to assist in the proper recognition of this injury pattern. A radial-head dislocation or subluxation should be suspected if the posterior border of the ulna on a true lateral radiograph deviates > 0.01 mm from a straight line.