Objective. To assess platelet function under defined flow conditions in patients with antiphospholipid syndrome (APS) and to correlate the results with thrombotic complications and the presence of subsets of antiphospholipid antibodies (aPL), lupus anticoagulant (LAC), and/or anticardiolipin antibodies (aCL). Methods. We studied 88 randomized APS patients with or without a history of thrombosis. Seventeen patients with other thrombosis (no APS) and 26 healthy subjects served as controls. Platelet adhesion and aggregation on the extracellular matrix were measured with a cone-and-plate(let) analyzer (CPA) by examining the percentage of total area covered with platelets (surface coverage [SC]) and the mean size of surface-bound objects (average size [AS]) and were compared with platelet responses to different ADP concentrations by conventional aggregometry. Results. Under defined flow conditions, SC and AS were significantly higher for venous thrombosis and arterial thrombosis in APS patients compared with no thrombosis, other thrombosis, and healthy control groups. The increased platelet adhesion and aggregation in APS patients with thrombotic events was associated with higher levels of von Willebrand factor (vWF) antigen (mean ± SD 230.6 ± 51.2%) and ristocetin cofactor activity (181.0 ± 36.0%). No change in CPA and vWF parameters was found in APS patients with positive results for aPL who did not undergo thrombotic events or in patients with other thrombosis. The CPA parameters were neither associated with the high response of platelets to ADP nor associated with the presence of LAC, aCL, or both. The CPA parameters were similarly increased irrespective of aspirin use. The results suggest that the increased adhesion properties of platelets in APS patients could be mediated by high levels and activity of vWF. This complements the known ability of APS antibodies to enhance platelet response to agonists in conventional aggregometry. Conclusion. The CPA test was found to be valuable in differentiating APS patients with and without thrombotic complications.