A follow-up examination of 21 cases of anterior interosseous nerve syndrome in 20 patients is presented. In 15 patients the anterior interosseous nerve was explored, with objective signs of compression in nine. 11 of these patients showed satisfactory function and three had a tendon transfer. Five cases were not operated on. Two recovered and three had continuous paralysis after more than four years. One patient had a primary tendon transfer. We conclude that exploration of the anterior interosseous nerve is the treatment of choice, and that expectant treatment can be reserved for patients with slight disability or poor general health. As recovery may take over a year, tendon transfer should be postponed until after this period in patients who do not show satisfactory recovery.