['Plasma levels of soluble P-selectin (sP-selectin) are often used to demonstrate platelet activation.', 'We determined sP-selectin in a variety of disorders characterized by high or low platelet counts and compared their levels with those in healthy subjects. Furthermore, we determined the Thr715Pro polymorphism in all subjects.', 'Total concentrations of sP-selectin were clearly associated with levels of platelet counts. Thus, calculation of sP-selectin per platelet showed that these levels in patients with thrombocytopenia due to marrow failure and in patients with increased platelet counts were similar to those in controls. Only patients with an increased platelet turn-over had elevated sP-selectin per platelet. While carriers of the Pro715 polymorphism had lower sP-selectin levels than non-carriers, this genetic disposition was over-ruled in patients with increased platelet turn-over.'] Are plasma levels of P-selectin determined by platelet turn-over and the P-selectin Thr715Pro polymorphism?