Purpose This study analyzes the outcomes after unilateral inferior oblique anterior transposition (IOAT) for manifest dissociated vertical deviation (DVD). Methods A retrospective chart review was conducted for all patients who had unilateral or markedly asymmetric DVD, ipsilateral overaction of the inferior oblique muscle, lack of alternating fixation, and underwent unilateral IOAT surgery between March 1997 and March 2001. In each case, the bunched inferior oblique muscle was anteriorly transposed to the lateral edge of the insertion of the inferior rectus muscle. The primary outcome variable was change in DVD. Secondary outcome variables included inferior oblique muscle action, graded from -4 to +4, and vertical deviation in primary gaze. Results Ten consecutive patients met the inclusion criteria. Median age at the time of surgery was 14 years (range, 2 to 41 years.) Mean follow-up was 25 months (range, 6 to 60 months). Ipsilateral DVD in primary position decreased from a mean of 20.2 prism diopters (PD) (range, 14 to 33 PD) to 3.7 PD (range, 0 to 9 PD) (t test, P < .001). Nine (90%) of the patients had an excellent postoperative result (residual DVD of 0 to 4 PD) and one (10%) had a good result (5 to 9 PD). Inferior oblique overaction was eliminated in all patients. Mean inferior oblique muscle action decreased from +2.4 to -1.3. Three patients developed a transient or permanent 4 to 5 PD postoperative ipsilateral hypotropia in primary position. Dissociated vertical deviation in the fellow eye did not develop, or if present preoperatively, did not increase. Conclusions Unilateral IOAT is an effective treatment for unilateral or markedly asymmetric DVD in patients with a strong, contralateral fixation preference. This surgery reduces inferior oblique overaction but may cause an ipsilateral hypotropia.