VWF is required for thrombus growth under venous shear conditions. (A) Time to platelet thrombus formation (30-150 μm in diameter) was measured. In the absence of VWF, the thrombi growth was impaired. The time to form the first 150-μm thrombus in VWF^−/− venules was nearly twice that in WT. (B) Rate of individual thrombus growth was determined in the VWF^−/− mice infused with r-hu-FVIII and compared with WT. Time 0 was considered when thrombus reached approximately 40 μm in size and thrombi were followed for 10 minutes. In VWF^−/− mice infused with r-hu-FVIII the thrombus grew significantly more slowly than in WT (*P < .05, **P < .001). Error bars indicate SEM.