OBJECTIVE To evaluate the clinical effectiveness and anatomic changes resulting from bupivacaine injection into extraocular muscles to treat comitant horizontal strabismus. METHODS Prospective, observational clinical series. METHODS Thirty-one comitant horizontal strabismus patients. METHODS Nineteen patients with esotropia received bupivacaine injections in the lateral rectus muscle, and 12 patients with exotropia received bupivacaine injections in the medial rectus. Sixteen of these, with large strabismus angles, also received botulinum type A toxin injections in the antagonist muscle at the same treatment session. A second treatment was given to 13 patients who had residual strabismus after the first treatment. METHODS Clinical alignment measures and muscle volume, maximum cross-sectional area, and shape derived from magnetic resonance imaging, with follow-up examinations for up to 3 years. RESULTS At an average of 15.3 months after the final treatment, original misalignment was reduced by 10.5 prism diopters (Δ; 6.0°) with residual deviations of 10Δ or less in 53% of patients. A single treatment with bupivacaine alone reduced misalignment at 11.3 months by 4.7Δ (2.7°) with residual deviations of 10Δ or less in 50% of patients. Alignment corrections were remarkably stable over follow-ups for as long as 3 years. Six months after bupivacaine injection, muscle volume had increased by 6.6%, and maximum cross-sectional area had increased by 8.5%, gradually relaxing toward pretreatment values thereafter. Computer modeling with Orbit 1.8 (Eidactics, San Francisco, CA) suggested that changes in agonist and antagonist muscle lengths were responsible for the enduring changes in eye alignment. CONCLUSIONS Bupivacaine injection alone or together with botulinum toxin injection in the antagonist muscle improves eye alignment in comitant horizontal strabismus by inducing changes in rectus muscle structure and length.