Abstract Background: Surgery for breast cancer can cause various deficiencies in upper limb and the surgery type can influence, normally the radical surgeries are more harmful. The presence of pain, reduced range of motion and decrease of muscle strength of the upper limb in the early postoperative period are some of the major deficiencies of these patients. The aim of this study was to evaluate muscle strength and range of motion of the shoulder in patients undergoing partial or total mastectomy to treat breast cancer preoperatively, 7 and 30 days after surgery. Methods: We included patients with breast cancer and surgical indication in a prospective study in the Gynecological Department of Sao Paulo Federal University, Brazil. Exclusion criteria were previous breast surgeries, immediate reconstruction, bilateral surgery, orthopedic diseases in the shoulder and neurological diseases. A total of 114 women were evaluated preoperatively (Pre), seven (PO7) and thirty days (PO30) after surgery, being 70 partial mastectomies (P) and 44 total mastectomies (T). For the assessment of muscle strength, it was used a Hand Held Dynamometer, model 01163 from Lafayette Instrument Company®, using the peak force in kilogram. It was calibrated to isometric contraction for five seconds. Each movement was measured twice and the average was used for the final rating. To measure the range of motion in degrees was used a goniometer. For both assessments cited the movements were: flexion, extension, adduction, abduction, internal and external rotation of the shoulder. Results: We used the test with two-factor ANOVA and multiple comparisons with the Tukey's test to analyze the results. In the assessment of muscle strength, every movement had a statistically significant difference between the periods Pre, PO7 and PO30 (p < 0.001). Thereby, a reduction of strength in PO7 improvement in PO30, but did not reach the initial period of Pre. The internal rotation, beyond the difference between the periods, showed a difference in muscle strength between the types of surgery (p = 0.005) being greater reduction in group T, on PO7 and PO30. In evaluating range of motion of shoulder, extension and internal rotation showed no statistical differences for periods or for the surgical groups. The flexion, abduction and adduction had statistical differences between the three periods (p < 0.001) and also between surgical types, always with smaller values of range of movements in group T compared to P (flexion p = 0.013; abduction p < 0.001 and adduction p < 0.001). However, the adduction in partial mastectomies had returned its values to the Pre period on PO30 (p < 0.001). And finally the external rotation showed difference only between the periods pre and post-surgery (p < 0.001), but not between PO7 and PO30. Conclusion: Surgery for breast cancer causes reduction in muscle strength of the shoulder that does not return after 30 days of surgery in both surgical types (P and T). It also reduces the range of motion of this joint in flexion, abduction, adduction and external rotation, with deficits of partial always lower than the total mastectomy. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P3-09-13.