A prospective longitudinal study of shoulder function after 103 neck dissections involving either preservation or sacrifice of the spinal accessory nerve is presented. The postoperative evolution and course of trapezius muscle denervation and resultant shoulder dysfunction were objectively determined for both radical and modified nerve sparing neck dissections. All patients were enrolled in a program of physical therapy aimed at maintaining range of motion at the shoulder joint. Shoulder function was examined preoperatively and for 12 months postoperatively with manual muscle strength testing, range of motion measurements, and electrodiagnostic testing. Results indicate that modified nerve sparing dissections are followed on the average by a significant, but temporary and reversible phase of shoulder dysfunction. By comparison, radical neck dissection is followed by profound and permanent trapezius muscle weakness and denervation.