Blood-borne tissue factor (TF)-bearing microparticles have been shown to play an important role in thrombus propagation in experimental models. The pathophysiologic role of these microparticles is being investigated in several prothrombotic conditions including cancer-associated thrombosis. Tumor cells are known to shed TF-bearing microparticles in vitro, and circulating TF-bearing microparticles can be measured in plasma samples from patients with advanced cancer. We are currently using an impedance-based flow cytometer to accurately size and enumerate TF-bearing microparticles to explore the association between cancer thrombosis and elevations in circulating TF-bearing microparticles.