The role of oxidative/reductive balance derangement in the pathogenesis of diabetic microangiopathy has often been discussed in the last few years. Therefore, we decided to evaluate the influence of intensive insulin therapy on selected indicators of free radical production. The levels of plasma hydrogen peroxide (H2O2) and serum malonyldialdehyde (MDA) were estimated in 15 patients with Type 1 and 15 with Type 2 diabetes before and after 2 weeks of intensive treatment. The initial H2O2 and MDA levels in all cases were significantly higher than in controls. After 2 weeks of treatment, the values for both estimated parameters were significantly lower; however, they were still higher than in the control group. Our results seem to confirm the previous suggestions concerning the relation between metabolic disturbances and oxidative stress in diabetic patients.