Background Strabismus surgery is a 1-day operation. It can result in aligned eyes or nearly so (orthophoria) or overcorrection or undercorrection that may require further treatment or another surgery. Aim To assess alterations in vertical palpebral fissure height following horizontal recti muscle surgery (recession or resection) for the treatment of horizontal strabismus (esotropia or exotropia). Patients and methods A prospective interventional case series was carried out on 68 eyes of 41 patients with horizontal strabismus (exotropia or esotropia), comitant or incomitant (paralytic), who underwent surgical correction. Patients were subdivided into three groups: group I (n=50) underwent recession only, group II (n=10) underwent resection only, and group III (n=8) underwent recession and resection. All patients were subjected to preoperative, operative, and postoperative evaluations (palpebral fissure height and width measurement). Results Distant angle of exotropia was considerably lower in group I than group III (P2<0.019) but were insignificantly different between groups I and II and between groups II and III. Palpebral fissure at 6 months was considerably lower in group II than groups I and III (P=0.013 and 0.019, respectively). Conclusion Horizontal rectus muscles recession for horizontal strabismus causes vertical widening of palpebral fissure height, and also resection results in narrowing of vertical palpebral fissure height. The combination of horizontal rectus muscle recession and resection for horizontal strabismus results in vertical palpebral fissure height widening in half of eyes and vertical palpebral fissure height narrowing in a quarter of the eyes.