The aim of this prospective study was to investigate the effects of early onset rehabilitation program on shoulder mobility, functional status, lymphedema, and the incidence of postoperative complications in patients who had modified radical mastectomy. Fifty-seven women were randomly assigned to either treatment group or home exercise program groups, which were consisted of 27 and 30 patients, respectively. After removal of the drains, the patients were instructed to 15 sessions of individual rehabilitation program and continued with home-based physical activity program. The home exercise program groups only received a form including the exercises, which could be performed by themselves after removal of the drains. Range of motion of the shoulder joint and upper extremity circumferential difference were measured. Functional status was assessed by functional index score. Each patient was assessed preoperatively and then postoperatively at fifth day and first, third, and sixth months. The improvement in measurements of flexion, abduction, and adduction movements of the shoulder joint and the functional questionnaire scores were significantly better in treatment group. There was no statistically significant difference in the development of lymphedema and postoperative complications in both groups. Early onset rehabilitation program after modified radical mastectomy provides improvement in shoulder mobility and functional capacity without causing adverse effect in postoperative period.