Fibrinogen substitution can correct bleeding in afibrinogenemia. We assessed the effect of fibrinogen substitution in a patient lacking immunoreactive fibrinogen. Fibrinogen and thrombin time were not measurable before, but became detectable within 30 min after substitution, parallelled by an increase in ADP-induced platelet aggregation from < 10% to 32%. Platelet adhesion, measured by Stagnation Point Flow Adhesio- Aggregometry, was not detectable prior to substitution but attained normal values thereafter. Scanning electron microscopy of adhering platelets revealed pseudopodia protrusion and spreading. Morphometry revealed two populations of spread platelets one of which demonstrated inhibited spreading as compared to healthy controls. Immunoelectron microscopy revealed normal GPIIb/IIIa receptor expression, both before and after substitution. Dynamic and kinematic viscosity of plasma and whole blood remained below the 99.9% confidence border of a healthy control group. In afibrinogenemia fibrinogen levels as low as 10% of normal concentration sufficed to normalize coagulation, platelet adhesion, and, partially, spreading.