To assess the potential for misdiagnosis of von Willebrand disorder (vWD) and hemophilia A while following current National Committee for Clinical Laboratory Standards (NCCLS) guidelines and consequent to a poorly recognized cold-activation phenomenon, we processed 39 normal citrate-anticoagulated samples by standard procedures (reference) or stored at low (approximately 4 degrees C) or ambient (approximately 22 degrees C) temperature for 3.5 hours before centrifugation and processing. Samples were tested in parallel for several hemostasis factors, including von Willebrand factor (vWF). Similar results were obtained for all samples for factors II, V, VII, IX, X, XI, and XII. For factor VIII (FVIII) and vWF, only samples stored at ambient temperature had results comparable to reference sample results. In most cases, low temperature storage led to much lower results. Taking the lower reference limit as 50%, most would have been defined as "abnormal," and a misdiagnosis of vWD or hemophilia A could easily arise. ABO classification and age were associated with FVIII and vWF levels, but neither was associated conclusively with relative loss of plasma FVIII coagulant and vWF caused by the cold-activation phenomenon. We advise laboratories following current NCCLS guidelines not to store or transport whole blood samples for FVIII and vWF testing at 2 degrees C to 4 degrees C because of the risk of misdiagnosing vWD or hemophilia A. Storage and transport at ambient temperature seem acceptable and provide results comparable to freshly centrifuged samples.