OBJECTIVE: To assess and compare the magnitude of lameness and level of pain after muscle-sparing lateral thoracotomy (MSLT) and standard lateral thoracotomy (SLT) in dogs. STUDY DESIGN: Randomized, blinded, prospective clinical study. ANIMALS: Twenty-eight client-owned dogs. METHODS: The latissimus dorsi muscle was retracted in the MSLT group and was transected in the SLT group. Gait was analyzed with a force plate, and the peak vertical force symmetry index (SI) was calculated within 24 hours before surgery, 3 days postoperatively, and 8 to 12 weeks postoperatively. Symmetry index and pain scores as measured by the Glasgow Composite Measure Pain Scale - Short Form were assessed as primary outcome measures. RESULTS: The SI 3 days postoperatively was lower compared with the preoperative SI value in all dogs, consistent with lameness of the ipsilateral thoracic limb (P < .001). The absolute differences in preoperative and 3-day-postoperative SI provided evidence that this change was 3.1-fold greater after SLT compared with after MSLT (P = .009). Pain scores 1 day after surgery were lower after MSLT (1) compared with after SLT (2.5, P < .001). CONCLUSION: Lateral thoracotomies caused postoperative pain and ipsilateral forelimb lameness, and both were reduced by sparing the latissimus dorsi. CLINICAL SIGNIFICANCE: Sparing the latissimus dorsi should be considered to decrease immediate postoperative morbidity in dogs undergoing lateral thoracotomy.