Objective: We investigated oxidative status in patients with rotator cuff tendinopathy (RCT) and evaluated their relationship with radiological and clinical parameters. METHODS: In this cross-section study, 88 patients with RCT (59 males and 29 females) and 86 healthy controls (66 males, 20 females) were enrolled. The sample consisted of nontraumatic patients who are suffering from shoulder pain because of rotator cuff disease, which was established by clinical tests and MRI scanning. Oxidative stress in patients with RCT was analyzed via the dynamic thiol/disulfide homeostasis (TDH). Thiol/disulfide homeostasis was measured by a new colorimetric method. Furthermore, oxidative stress was indirectly measured by serum total oxidant status (TOS), oxidative stress index (OSI), and total antioxidant capacity (TAC). Results: Serum disulfide levels and the other oxidative stress parameters of the RCT group were significantly greater than those of the control group (P < .001 for all), whereas the anti-oxidative stress parameters remained unchanged (P > .05 for all). The lowest and highest serum disulfide levels were detected in patients with grades 1 and 3, respectively (P < .001). Furthermore, in a multiple regression analysis, the disulfide/natural thiol ratio (β=–4.886, P = .004) and the MRI grading (β=0.314, P=.001) were independently associated with the Western Ontario Rotator Cuff Index WORC score. Conclusion: We found an association between the levels of various serum markers of oxidative injury, especially serum disulfide levels, and the increasing severity of RCT. Thiol/disulfide homeostasis seems to play a critical role in RCT, both in the beginning and during the progression of disease.