We evaluated real-time processes of platelet thrombus formation on a collagen surface in a flow chamber with whole blood from patients with various platelet aggregation disorders, such as Bernard-Soulier syndrome (BSS), Glanzmann’s thrombasthenia (GTA), type 3 von Willebrand disease (vWD), and congenital afibrinogenemia (Af), who lack platelet glycoprotein (GP) Ib-IX complex, GP IIb-IIIa, von Willebrand factor (vWF), and fibrinogen, respectively. Blood from GTA patients showed impaired thrombus growth but significant initial platelet-surface interaction under all shear conditions tested (50 to 1,500 s⁻¹). By contrast, blood from patients with BSS or type 3 vWD showed no platelet-surface interaction under high shear (≥1,210 s⁻¹) but normal thrombus formation under low shear (≤340 s⁻¹). When shear rate was increased stepwise to 1,500 s⁻¹ during perfusion, the thrombus growth observed in type 3 vWD or BSS under low shear was arrested, whereas that in control blood was sharply accelerated as a function of shear rate. Overall thrombus formation in Af appeared indistinguishable from that of a control under shear rates between 50 and 1,500 s⁻¹. However, Af thrombi formed under such conditions collapsed immediately when shear rate was further increased to 4,500 s⁻¹, whereas thrombi of type 3 vWD or BSS formed under low shear were stable even when shear rate was elevated to 9,000 s⁻¹ during perfusion. These findings suggest that distinct molecular mechanisms underlie the pathologic bleeding in these diseases and point to the distinct roles of two major adhesive proteins, vWF and fibrinogen. In mural thrombus formation under flow conditions, vWF, perhaps mainly through its interaction with GP Ib-IX, acts as an “initiator and promoter,” whereas fibrinogen, via its binding to GP IIb-IIIa, acts as a “stabilizer” against heightened shear forces that could lead to peeling off of platelets from the surface.