PURPOSE: To evaluate postoperative binocularity after surgical correction of longstanding constant-horizontal adult strabismus without both stereopsis and previous surgery. METHODS: Subjects included 43 adults over 18 years of age, who had constant-horizontal strabismus for more than 10 years without previous surgery and stereopsis. The patients were composed of 30 exotropes and 13 esotropes. Patients with a corrected visual acuity less than 20/40 in the deviating eye were excluded. After correcting for refractive error, the angle of deviation was measured, and pre- and postoperative stereopsis was assessed with the Titmus fly test and Randot stereo test. The relationship between duration of deviation and recovery of stereopsis or obtained stereoacuity were evaluated. RESULTS: The mean preoperative angle of deviation was 49.8+/-12.56 PD in exotropes and 34.7+/-12.41 PD in esotropes. All patients showed suppression of the deviating eye at distance and near before surgery. Postoperatively all patients showed orthophoria within +/-8 PD. Stereopsis was obtained in 80% of exotropes and in 30.8% of esotropes. The mean postoperative stereoacuity was 449.2+/-815.29 seconds of arc (sec) in exotropes and 1000+/-1336.66 sec in esotropes. However, there was no significant relationship between the duration of deviation and the obtained stereoacuity (p>0.05), or between the duration of deviation and the recovery of stereopsis (p>0.05). CONCLUSIONS: Adults with longstanding constant-horizontal strabismus more than 10 years obtained stereopsis with good quality after surgical correction. The recovery of stereopsis was much better in exotropes than in esotropes. Recovery was not influenced by the duration of deviation.