Background -Weakening of the inferior oblique muscle is the procedure of primary importance in the treatment of patients with superior oblique palsy, Knapp's classes I and III. In this study, the effectiveness of anterior transposition of the inferior oblique muscle in the treatment of these patients was evaluated. Methods -Twenty three patients with superior oblique palsy, Knapp's classes I and III underwent anterior transposition of the inferior oblique muscle. These patients had 10 to 25 prism diopters (PD) hyperdeviation in primary position. The tip of the disinserted muscle was sutured to the sclera parallel and adjacent to the lateral border of the inferior rectus muscle insertion. The prism and alternate cover test measurements were performed in all cardinal positions of gaze before and six months after surgery. Results -The mean reduction of hyperdeviation measured 14.9 PD in the primary position, 23 PD in adduction, 25.2 PD in elevation and adduction, and 18.1 PD in depression and adduction. There was no hypotropia in the primary position or limitation of upgaze. Postoperative hyperdeviation in the primary position was 5 PD or less in 21 out of 23 patients. Conclusion -The anterior transposition of the inferior oblique muscle is very effective in eliminating hyperdeviation in patients with superior oblique palsy, Knapp's classes I and III. Up to 25 PD of hyperdeviation reduction in the primary position can be achieved. If this type of anterior transposition is used, hypotropia in the primary position or limitation of upgaze will possibly not occur.