Background The pathogenesis of psoriasis has not been known exactly yet. Recently, it has been suggested that increased reactive oxygen species (ROS) such as nitric oxide (NO) and malondialdehyde (MDA) may play a part in the pathogenesis of various skin diseases, including psoriasis. Objectives In this study, we aimed to investigate the role of ROS in the pathogenesis of psoriasis. Methods A convenience sample of 23 patients with psoriasis and 23 healthy subjects consented to participate in the study. Plasma NO and MDA levels were measured in all participants. Psoriasis area and severity index (PASI) and tissue levels of MDA on lesional and non-lesional skin regions of psoriasis patients were measured. In addition, the correlation between age, gender with plasma NO, plasma MDA and tissue MDA was assessed. Results Plasma levels of NO and MDA in psoriasis patients (135.8 mu mol/L, 4.33 mu mol/L, respectively) were statistically significantly higher than those in controls (33.6 mol/L, 2.03 mu mol/L, respectively). Tissue levels of MDA in lesional tissues (49.18 nmol/gr) were significantly higher than those in non-lesional tissues (28.41 nmol/gr). A significant correlation was not found between the PASI and levels of NO and MDA. In addition, a significant negative correlation was found between the plasma NO levels and age. Conclusion NO and MDA levels are elevated in psoriasis patients, which may indicate that oxidative stress plays an important role in the aetiopathogenesis of psoriasis.