This study attempted to verify the existence of a relationship between oxidative stress documented by malondialdehyde (MDA) and superoxide dismutase (SOD) and fibrinolysis analysed by tissue plasminogen activator (tPA) and its inhibitor (PAI-1) in diabetes mellitus. Forty-seven patients with Type 1 (n = 27) and Type 2 (n = 20) diabetes were examined together with 20 non-diabetic controls. The following were analysed: plasma MDA concentration, SOD activity in erythrocytes, tPA activity and antigen, PAI-1 activity and antigen, fasting blood glucose, fructosamine, glycated haemoglobin (HbAlc), and urine albumin. SOD activity was decreased in patients with diabetes. This contrasted with an increased plasma MDA concentration especially in Type 2 diabetes as compared with Type 1 or healthy persons (p < 0.001). tPA activity was increased in both groups of patients with diabetes as compared to healthy persons (p < 0.001), PAI-1 activity was higher in Type 2 diabetes with vascular changes than in the remaining subgroups (p < 0.001). Multivariate analysis revealed a significant positive relationship between plasma MDA concentrations and PAI-1 antigen (r = 0.53, p < 0.001) and a negative relationship between SOD and tPA activities (r = -0.53, p < 0.01). We conclude that oxidative stress may modulate fibrinolytic properties in diabetes mellitus.