Thrombomodulin (TM), a marker of endothelial cell damage was studied in 183 patients with diabetes mellitus and different stages of complications. Thrombomodulin plasma levels correlated with duration of diabetes in patients with type I and type II diabetes. Thrombomodulin levels were higher in patients with increasing numbers of complications (nephropathy, retinopathy, arterio-occlusive disease, neuropathy). Neither the presence of retinopathy, nor neuropathy alone significantly increased plasma thrombomodulin in patients with similar urinary albumin concentration. The plasma level of thrombomodulin was more prominent in hypertensive than normotensive patients. Multivariate analysis showed that albuminuria is the factor which influences the most the increase of thrombomodulin in serum of diabetic patients.