OBJECTIVES: Platelet activation, which may lead to thrombus formation, is poorly understood after valve surgery when analysed by flow cytometry. It is still unclear whether platelet activation is due to the implanted prosthesis itself or to its aortic or mitral position. The present prospective study was aimed at assessing platelet P-selectin expression, platelet-leukocyte conjugate formation and platelet microparticles and comparing biological, bileaflet and tilting disc valvular prostheses in aortic position. METHODS: Thirty-three patients were included (15 bioprostheses, 11 bileaflet and 7 tilting disc mechanical valves). Blood samples were analyzed by flow cytometry immediately before surgery, on postoperative day 8 (D8) and 2 months after surgery (M2). RESULTS: At D8, patients with bileaflet valves demonstrated a significant increase in platelet-leukocyte conjugates and in platelet microparticles whereas only platelet-monocyte conjugates were significantly increased in patients with bioprostheses. At M2, platelet activation had returned to the basic level observed prior to surgery in the bileaflet valve group whereas, it was still increased in the bioprosthesis group. Neither at D8 nor at M2 did patients with tilting disc mechanical valves demonstrate any platelet activation. CONCLUSIONS: Platelet activation after aortic valve replacement was observed with the use of bioprostheses and bileaflet mechanical valves, but not with tilting disc mechanical valves.