Kocak I, Hizmetli S, Tas A, et al. - The aim of this study was to compare the serum cathepsin S (CatS) and cystatin C (CysC) levels between patients with FM and healthy control subjects. Between January 2019 and October 2019, this study was performed in the Physical Medicine and Rehabilitation Clinic. Researchers recruited a total of 145 fibromyalgia syndrome (FM) patients newly diagnosed with primary FM according to the 2010 American College of Rheumatology criteria and 129 healthy volunteers. They noted age, gender, and BMI of the participants. They obtained venous blood samples from both groups for the assessment of the levels of serum CatS and CysC. The Fibromyalgia Impact Questionnaire was applied to assess the functional status of FM patients. This study's findings demonstrated that serum CatS and CysC levels were higher in FM patients. Nevertheless, no association was found between the functional status of FM patients and serum CatS and CysC levels. These outcomes can be of guidance for future clinical studies of the etiopathogenesis and treatment of FM.