Although platelets may contribute to the inflammatory component in atrial fibrillation (AF), the impact of platelet-leukocyte mixed conjugates has not yet been determined. Seventeen patients with persistent AF (8/9 m/f; mean age 68.1 +/- 2.5 years), not on anticoagulant therapy, were recruited and compared to 34 healthy controls with normal sinus rhythm (16/18 m/f; mean age 60.8 +/- 1.2 years). Platelet-leukocyte mixed conjugates, platelet P-selectin and leukocyte activation markers (CD11b, myeloperoxidase) were measured by flow-cytometry in whole blood both in basal condition and after in vitro ADP/collagen challenge. Plasma D-dimer and soluble P-selectin were also measured. Statistical analyses were performed by Mann-Whitney or Wilcoxon U test for intergroup differences. In AF patients platelet count, as well as platelet P-selectin expression and percent platelet-leukocyte conjugates were all significantly lower both in basal condition and upon activation with ADP/collagen. In contrast, both soluble P-selectin and D-dimer were significantly higher than in controls; white blood cell count and leukocyte activation markers were unchanged. In conclusion, the formation of platelet-leukocyte mixed conjugates was unexpectedly reduced in AF, possibly due to less reactive platelets as a consequence of previous in vivo activation by ongoing formation of trace amounts of thrombin.