Purpose-To examine the effect of preoperative alignment stability on postoperative motor outcomes in children who underwent surgery for esotropia.Methods-167 subjects (68 with infantile esotropia and 99 with acquired esotropia) aged less than 6 years had surgery after completing 18 weeks of follow up as part of an observational study. Preoperative alignment was classified as stable, uncertain, or unstable, based on measurements taken at baseline and every 6 weeks for 18 weeks. If the absolute value of the difference between the largest and smallest of the four measurements was within 5 PD inclusive, preoperative alignment was classified as stable; if the difference was 15 PD or greater, alignment was classified as unstable; and if neither of these criteria were met, alignment was classified as uncertain. Distance alignment measured by prism and alternate cover test was compared among stability classification groups at 6 weeks and 6 months after surgery.Results-Among subjects with infantile esotropia, median 6-week postoperative deviation was 2 prism diopters (PD), 6 PD, and 2 PD for subjects with stable, uncertain, and unstable preoperative alignment, respectively (P = 0.73 for stable vs. unstable). Median 6-month postoperative deviation was 1 PD, 9 PD, and 1 PD for stable, uncertain, and unstable, respectively (P = 1.00 for stable vs. unstable). Among subjects with acquired esotropia, median 6-week postoperative deviation was 6 PD, 4 PD, 4 PD for subjects with stable, uncertain, and unstable preoperative alignment, respectively (P = 0.69 for stable vs. unstable). Median 6-month postoperative deviation was 8 PD, 4 PD, 6 PD, for stable, uncertain, and unstable, respectively (P = 0.22 for stable vs. unstable).