Retrospective studies indicate that only 50% of surgical patients with acquired esotropia achieve good eye alignment with one surgery. New evidence indicates that with the preoperative use of prisms, good eye alignment can be achieved with one operation in approximately 85% of surgical patients with acquired esotropia. A multicenter, randomized, prospective, clinical trial with careful attention to standardization and unbiased outcome evaluation is proposed to determine if: 1. The surgical result in patients with acquired esotropia is better if they have preoperative prism adaptation than if they do not have preoperative prism adaptation. 2. If those patients who respond to prism adaptation are more accurately corrected by operating for the prism adapted angle or the original angle of deviation. 3. If there are clusters of input variables, (e.g. age, size of deviation, sensory status, refractive error, or AC/A ratio) that help to predict which patients are more likely to respond to the prism adaptation test. In an effort to answer these questions a double randomization study has been designed. Two-thirds of the patients will be randomly selected to go through prism adaptation prior to surgery. One-third of the patients will have surgery based on the amount of crossing measured using routine examination techniques. Of those patients who respond to the prisms, one-half will have surgery based on the amount of prism required to