von Willebrand's factor is required for platelet adhesion to subendothelium, and for normal factor VIII survival in the circulation. These functions require the assembly of von Willebrand's factor into multimers that exhibit properly regulated binding to platelet glycoprotein lb. Recent studies suggest that the propeptide of von Willebrand's factor may catalyze multimer assembly and have identified new segments of von Willebrand's factor that appear to regulate its affinity for glycoprotein lb. Two segments of von Willebrand's factor have been found to interact with collagen type VI, which is a candidate binding site for von Willebrand's factor in the subendothelium. Advances in the identification of mutations have prompted a reclassification of von Willebrand's disease. ABO antigens on von Willebrand's factor may impair the efficacy of plasma or recombinant von Willebrand's factor when administered to patients with incompatible ABO blood type.