We report the renal immunohistochemical findings of a patient with thrombotic thrombocytopenic purpura complicated with systemic lupus erythematosus. Aggregated platelets were observed adhering to the arteriole walls. Intense deposition of von Willebrand factor (vWF) was noted in the endothelium, even in areas where thrombi were not seen. This difference in the distribution of platelets and vWF suggested that the endothelium was damaged prior to platelet aggregation.