Human von Willebrand Factor (vWF) was studied by atomic force microscopy under physiologic buffer on a hydrophobic octadecyltrichlorosilane self-assembled monolayer. The self-assembled monolayer deposited on glass was sufficiently smooth (root mean square roughness = 0.25 +/- 0.12 nm) to permit identification of adsorbed vWF. Adhesion of the protein to the hydrophobic substrate was sufficient to allow repeated scanning by the atomic force microscope probe, and images of vWF on a submolecular scale were obtained. The frictional force between the surface and the protein was sufficient to withstand an applied lateral force of 19 nN. This result shows that vWF experiences strong interaction with a hydrophobic surface in aqueous media. Statistical analysis of adsorbed vWF shows that the protein is composed of large globular domains with elliptical cross sections of average dimensions 56 +/- 24 nm (major axis) 26 +/- 19 nm (minor axis), and 2.8 +/- 1.0 nm (height). Further analysis of the major axis dimension shows that the molecular chain of vWF contains two statistically different populations of domain size. However, no sequence order of the different domains within the individual molecule was found. On the basis of our analysis of the globular domains, we present a model describing the three-dimensional structure of vWF protomer adsorbed on a hydrophobic surface in a physiologic solution.