We developed a new system which enables direct observation of platelet adhesion on opaque biomaterials under shear flow conditions, by combining a thin quartz cone which produces laminar shear flows, with an upright epifluorescence microscope which visualizes stained platelets through the rotating cone. This is the first report on the observation of platelets adhered to opaque biomaterials in real time under shear flow conditions. The direct observation of platelet adhesion to expanded polytetrafluoroethylene (ePTFE) as an opaque biomaterial revealed that the kinetics of platelet adhesion to ePTFE depended greatly on shear stresses, showing that the shear stress of 5.0 dyne/cm2 induced higher adhesiveness of platelets to ePTFE than that of either 0.1 or 15 dyne/cm2. The observation also showed a difference in platelet adhesiveness among ePTFEs with different fibril lengths--0, 3.2, 18, and 35 microm--indicating that ePTFEs with shorter fibril length had lower adhesiveness of platelets under a shear stress of 5.0 dyne/cm2. It is indispensable for analyzing the phenomena of platelet adhesion to opaque biomaterials to observe in real-time rolling, adhesion, and detachment of platelets under shear stresses without disturbing shear flow conditions. The results showed that the mechanical and optical design of the system could serve this purpose.