Cancer patients experience several complications during the course of their illness which increase their morbidity and mortality. Thromboembolic phenomena are frequent and important events encountered in these patients. Although the exact mechanisms of thrombosis in this population of patients are largely unknown, speculations regarding the pathogenesis of clotting are presented in this short review. Interactions between elements related to the underlying neoplastic disorder, adherence to the vessel wall by cancer cells, indirect injury of the endothelial tissue, cytokine released by the malignant clone and disturbances in the clotting cascade to mention a few, are thought to be responsible for hypercoaguability in patients with cancer. More experimental and clinical studies are needed to clarify the etiology and management of these serious complications and concomitant diseases. Thrombotic complications are considered the second most common cause of death in patients with underlying malignant disorders. It has been estimated that 15% of patients with cancer will suffer from thromboembolic phenomena during their life (1,2). However, prospective data that examine the exact incidence of these complications are currently unavailable. Few investigators have estimated that the prevalence of serious thrombosis may be up to 50% in post mortem analysis (3).