Summary Background Fibromyalgia syndrome (FMS) is characterized by altered pain perception with chronic, widespread musculoskeletal pain. The relationship between nitric oxide, oxidative stress and the severity of FMS has not been studied. This study evaluated NO levels in plasma, LPO products and antioxidants in Red Cell lysate in patients of FMS and correlated it with disease severity. Methods 105 FMS patients who fulfilled 1990 ACR Criteria and 105 age- and sex-matched healthy controls were recruited over two years from 2013 to 2015. Antioxidative enzyme activity was assessed by the estimation of catalase, glutathione peroxidase (GPx) and glutathione reductase (GR) and superoxide dismutase (SOD). Nitric oxide in plasma, MDA marker of lipid peroxidation (LPO) in the lysate was donen for estimating oxidative stress. FIQR was used to assess the severity of fibromyalgia. Results The catalase, superoxide dismutase, glutathione reductase and glutathione peroxidase levels were significantly low in patients than controls (p<0.001). Plasma NO levels and LPO were also significantly high (p<0.05). NO and LPO levels showed a significant positive correlation with FIQR (r: 0.57, 0.8 and p: <0.001) whereas a negative correlation was observed between antioxidants (Cat, GR and GPx, but not SOD) and FIQR. Conclusions Low antioxidants and raised LPO in RBC lysate in patients with FM together with high plasma NO correlated with the severity of FMS.