We performed a prospective study of preoperative prism adaptation in 77 patients with acquired esotropia. Sixty-three of them increased their angle of squint when wearing Fresnel press-on prisms for 5-7 days. After the angle had stabilized to a point that did not exceed the press-on prisms by more than 10 prism D, they were randomly divided into two groups. Thirty-two patients underwent surgery based on the prism-adapted angle. The other 31 patients underwent surgery based on their initially measured angle. Fourteen patients who did not respond to prism correction underwent surgery based on the angle before prism correction. Success rates with deviations between 0 and 10 prism diopters measured 1 year after surgery were highest in those in whom surgery was based on the prism-determined angle and were lowest in the nonresponders, who had no fusion response to the prisms.