Forty patients with psoriasis and forty healthy controls were enrolled to study the levels of total blood thiols (an important part of the body's antioxidant defence system) and plasma malondialdehyde (a lipid peroxidation product). The levels of total blood thiols in psoriatic patients in the acute phase were significantly lower than those in controls, but total blood thiol levels in psoriatic patients in the remission phase were not significantly different from those in controls. There was a significant difference between levels of total blood thiols of patients in the acute phase and in remission. The levels of plasma MDA were significantly raised in psoriatic patients in the acute phase as compared to those in controls and in patients in remission. The differences in levels of plasma MDA were not significant between control subjects and patients in remission. These findings suggest a role of oxidative stress in the etiopathogenesis of psoriasis.