OBJECTIVE - The high risk for cardiovascular disease in IDDM patients with nephropathy may be mediated by abnormal function of the vascular wall. We investigated whether heparin was able to modulate markers of vascular wall and hemostatic function in patients with incipient nephropathy. RESEARCH DESIGN AND METHODS - Thirty-five IDDM patients with incipient nephropathy were randomized to treatment with placebo, unfractioned heparin, or low molecular weight heparin in a double-blind trial. The treatment was given as 1 h of conventional intravenous high-dose treatment and in a conventional subcutaneous low-dose regime for 3 months. Transcapillary escape rate of albumin and plasma levels of von Willebrand factor, fibrinogen, prothrombin fragment 1 + 2, thrombin-antithrombin III complexes, tissue type plasminogen activator, tissue plasminogen activator inhibitor type 1, total cholesterol, HDL cholesterol, and triglycerides were measured before and after treatment. Of the patients, 31 completed the study. RESULTS - We found no significant effect of heparin on markers of vascular wall and hemostatic function by any of the treatments. CONCLUSIONS - Treatment with high- or low-dose heparin induced no modulation of markers of vascular wall or hemostatic function in IDDM patients with incipient diabetic nephropathy.