We investigated the effects of high plasma lipid levels on platelet adhesion and platelet thrombus formation in nonanticoagulated human blood on collagen fibrils at an arterial wall shear rate of 2600 seconds' 1. Nonanticoagulated blood was drawn directly at a flow rate of 10 mL/min for 3 minutes from an antecubital vein of patients with type Ha (n=5) and type lib (n=4) hyperlipoproteinemia over purified human type III collagen fibrils that were positioned on a plastic coverslip in a parallel-plate perfusion chamber. Results were compared with those obtained in healthy individuals with normal lipid plasma levels (n=9). Blood-collagen interactions were quantified by morphometry as platelet-collagen adhesion, thrombus volume, and fibrin deposition. Platelet-collagen adhesion in the two groups of patients was significantly higher than in healthy individuals (70.7 [61.2 to 82.0] and 70.3 [66.4 to 81.0] in types Ila and lib patients, respectively, versus 51.2 [44.5 to 68.6] in control subjects; p<.05. All values are percent median [range]). In contrast, the thrombus volume was similar in the three groups (11.3 [8.0 to 13.0], 9.6 [6.4 to 15.3], and 10.2 [6.8 to 16.1] /tn$/fim1 [range], respectively). Differences in fibrin deposition were not observed. Thus, it appears that platelet-collagen adhesion is augmented in patients with type Ila and lib hyperlipoproteinemia, indicating that the process of thrombogenesis is hastened in these patients.