ImportanceRecovery of shoulder function following breast cancer surgery is crucial for physical functioning and quality of life. While early implementation of shoulder rehabilitation exercises may enhance recovery, the optimal timing and exercise program remain unclear.ObjectiveTo investigate whether an early exercise intervention, initiated 1 day postsurgery and continued for 1 month through subsequent visits, could improve shoulder range of motion (ROM) and strength in patients with breast cancer.Design, Setting, and ParticipantsA parallel-group, 2-arm randomized clinical trial was conducted between June 2020 and October 2021 at the Breast Cancer Center in Seoul, South Korea. Fifty-six patients (of 119 screened) with early-stage breast cancer who were scheduled for partial or total mastectomy were randomized into a tailored resistance exercise group (n = 28) or a usual care group (n = 28). Data were analyzed from November 2021 to June 2022.InterventionsThe exercise intervention commenced 1 day postsurgery and consisted of 4 supervised exercise education sessions corresponding with surgeon visits and daily home-based exercises for the first postoperative month. Tailored programs, including stretching and strength exercises, were adjusted based on individual shoulder function recovery status.Main Outcomes and MeasuresPrimary end points were shoulder ROM and strength at 1 and 6 months postsurgery. Physical activity, body composition, and quality of life were assessed at 6 months.ResultsOf 56 patients randomized (mean [SD] age, 50.3 [6.6] years), 54 completed the trial (96%), with 100% and 97% compliance to supervised and home-based exercise sessions, respectively. At 1 month postsurgery, 19 (67.9%) in the exercise group had fully recovered shoulder strength compared to 1 (3.6%) in the usual care group (P < .001). At 6 months, 22 (78.6%) in the exercise group had fully recovered shoulder ROM and 24 (85.7%) had fully recovered strength compared to 6 (21.4%) and 5 (17.9%), respectively, in the usual care group (P < .001). The exercise group exhibited less loss in muscle mass and improved physical activity and quality of life compared to the usual care group.Conclusion and RelevanceIn this trial, 1-month tailored exercise program, initiated immediately after breast cancer surgery and supplemented with supervised sessions coinciding with surgeon visits, significantly improved shoulder function in patients with breast cancer.Trial RegistrationWHO International Clinical Trials Registry identifier: KCT0006997