A new, automated assay for von Willebrand factor (vWF) activity has recently become commercially available (HemosIL vWF activity assay, Instrumentation Laboratories, Lexington, MA). We prospectively studied 61 specimens from 58 patients undergoing laboratory testing for suspicion of von Willebrand disease with this new method, in comparison with the established ristocetin cofactor method. Assays for factor VIII and vWF antigen were also performed using an established method on an MDA-180 coagulation analyzer (bioMérieux, Durham, NC) and a new method on an ACL TOP coagulation analyzer (Instrumentation Laboratories). Blood types were determined. The results showed no significant difference between the assays for factor VIII (mean, 97% for MDA-180 and ACL TOP; P = .494) or vWF antigen (mean, MDA-180, 109%; ACL TOP, 111%; P = .766). The mean result for the ristocetin cofactor assay was 106% vs 93% with the automated vWF activity (P = .007). The automated activity assay was 100% sensitive and 86% specific for detecting vWF abnormalities and seems to be a suitable screening test. Abnormal results should be followed up with a ristocetin cofactor activity assay for confirmation. Further study is recommended to confirm these conclusions.