Purpose/Hypothesis: Macromastia can be a morbid condition experienced by women who exhibit symptoms including shoulder pain, back pain, intertrigo, shoulder/bra strap grooving, and loss of function. However, efforts to objectify the medical necessity for reduction mammaplasty have been difficult. Physical therapists, with their unique knowledge of biomechanics and disablement, can play an important role in the justification of surgical interventions for these women. Therefore, the purpose of this study was to investigate the effect of reduction mammaplasty on net joint forces at the low back and function of patients with macromastia. Number of Subjects: 11. Materials/Methods: Eleven females (mean age: 44.18 +/- 12.88, range: 27-71y) determined to need a medically necessary reduction mammaplasty participated in a biomechanical analysis and responded to a questionnaire prior to and following their surgery (within one week of their scheduled surgery date and approximately 3 months [mean days: 87.09 +/- 13.7] following their surgery). Biomechanical analysis included performance of 3-trials of a constrained lifting task using a 5 lb. weight located 12 in. anterior to the feet and 16 in. above the ground. Concurrent kinematic and kinetic data from the task was collected using a 6-camera motion capture system (Vicon; Centennial, Co) and an in ground force-plate (AMTI; Watertown, MA). A custom made application (Matlab; Natick, MA) was used to calculate the net joint force at L5-S1 using an inverse dynamic approach based on Zatiorski's anthropometric data. Functional analysis consisted of the completion of the standardized Function Rating Index (FRI) questionnaire to assess their self-reported ability to perform dynamic movements of the neck and back and/or withstand static positions. Results: The average maximum total axial-force at the L5-S1 vertebral level during the lifting task decreased 9% post surgery (pre-test: 866.65 lbs +/- 156.69, post-test: 790.42 lbs +/- 134.18; p<0.025). Furthermore, scores on the functional rating index improved 76% (pre-test: 18.27 +/- 6.9, post-test 4.36 +/- 4.36; 0=Best Condition, 40=Worst Condition; p<0.002). Conclusions: Individuals who underwent reduction mammaplasty experienced decreased biomechanical stress in the low back. In addition, participants reported dramatic improvements in their ability to perform dynamic movements and withstand static positions. Clinical Relevance: Women with macromastia often face difficulty in exercising, potentially worsening or contributing to obesity and poor self-image. Physical therapists play an important role in the treatment of these individual problems and in characterizing the beneficial effects of reduction mammaplasty. Future research should examine the effect of physical therapy interventions combined with surgical treatment.