PURPOSE To investigate quantitatively the retinal vascular diameter changes before and after strabismus surgery, analyzing the potential hemodynamic changes that may occur in the retinal circulation. METHODS Consecutive patients with horizontal strabismus who underwent strabismus surgery were prospectively enrolled. Color fundus photographs were taken of each eye before and 1 day after surgery. The retinal vessel diameters were measured using computer-assisted quantitative assessment software. Paired-sample t test was used to evaluate the central retinal arteriolar equivalent (CRAE), central retinal venular equivalent (CRVE), and arteriovenous ratio (AVR) before and 1 day after surgery. We also analyzed those changes in different subgroups according to surgical protocols. RESULTS A total of 217 eyes of 148 patients were included. Compared with the data before surgery, the mean CRAE significantly increased 1 day after surgery (P = 0.01), so did the AVR (P = 0.003). There was no significant difference with respect to CRVE (P = 0.43). The CRAE and AVR were not significantly different in the lateral rectus recession group. CONCLUSIONS Strabismus surgery on horizontal rectus muscles may change retinal hemodynamics by increasing the central retinal arteriolar diameter during the early postoperative period.