Two families are described with members who have both von Willebrand's disease and telangiectasias. Family A has four members in three consecutive generations that have both von Willebrand's disease and telangiectasias. von Willebrand's disease in this family is characterized by decreased ristocetin cofactor (FVIII-vWF), variably depressed factor VIII coagulant (FVIII-AHG), and factor Vlll-related antigen (FVIII-AGN) levels. FVIII-AGN mobility on two-dimensional crossed immunoelectrophoresis was found to be normal. Four generations in Family B have von Willebrand's disease characterized by decreased FVIII-AHG, FVIII-vWF, FVIII-AGN, and prolonged template bleeding times. Two members of this family also have telangiectasias and recurrent gastrointestinal bleeding. Results in these two families suggest an association between von Willebrand's disease and telangiectasia—perhaps a defect in vascular endothelial cell function.