Olson, John D., Brockway, William J., Fass, David N., Magnuson, Mark A., and Bowie E. J. Walter: Evaluation of ristocetin–Willebrand factor assay and ristocetin-induced platelet aggregation. Am J Clin Pathol 63: 210–218, 1975. Normal subjects, patients with various bleeding disorders, and patients with von Willebrand’s disease were studied. All patients with von Willebrand’s disease had decreased levels of ristocetin–Willebrand factor (range, 0 to 41%) as compared with all other subjects (range, 79 to 202%). Ristocetin-induced platelet aggregation of platelet-rich plasma was abnormal in all patients with von Willebrand’s disease tested, and it was possible to correct this abnormal response by addition of normal platelet-poor plasma. Abnormal ristocetin-induced platelet aggregation was seen in patients with intrinsic platelet disorders or, on some occasions, in normal patients who had ingested aspirin. Ristocetin-induced platelet aggregation is not diagnostic, but it may be useful as a simple screening test for patients with possible von Willebrand’s disease. In conjunction with other tests, the assay for ristocetin–Willebrand factor will be useful in diagnosis and evaluation of these patients.