The posterior border of the ulna is the most important bony landmark for all dorsal surgical approaches and the guideline for open reduction, internal fixation of displaced comminuted fractures of the proximal ulna. We examined 74 cadaveric specimens to evaluate the anatomy of the proximal ulna, especially the course of the posterior border, the point of varus angulation, the width of the shaft, and the relationship of the posterior border to the interosseous and anterior ones. In 63 specimens, the mean point of varus angulation was 85.4 mm, and the average angulation angle was 17.7 degrees. In 11 specimens, the posterior border was either radially bowed or the bow was poorly defined and not localized to a specific area along the length of the bone. In conclusion, the variations of the proximal ulna have to be considered if dorsal plates and intramedullary screws are used.