The aim of this study is to assess the effectiveness of intraoperative botulinum toxin A (BTA) injection as an adjunct to the surgical treatment of large-angle esotropia or exotropia. Ten patients were included in the study. Mean age of the patients was 27+/-20 years. Of these 10 patients, 7 were esotropic and 3 exotropic. The average preoperative esodeviation was 73.6+/-16.5 prism diopters and exodeviation was 76.7+/-5.8 PD. Five units of BTA were injected intraoperatively into one of the recessed horizontal rectus muscles in all of the patients. The average follow-up was 14+/-10 months (range, 8 to 40 months). The average final deviation in the esotropia group was 13+/-9.6 PD of esotropia. The average final deviation in the exotropia group was 4.7+/-5 PD of exotropia. The final deviation of the 70% patients was within 10 PD of esotropia or exotropia. The results of this study suggest that the combination of BTA injection with recession may increase the expected correction of a conventional horizontal rectus muscle surgery.