Thrombotic events associated with antiphospholipid (aPL) antibodies can be the first manifestation of malignancy. The pathologic significance of aPL antibodies in patients with malignancies, however, is still unclear. It remains an unresolved issue as to whether the presence of aPL antibodies may be considered as an "epiphenomenon" of the malignancy itself or whether it contributes directly to the development of thrombosis in these patients. In particular, solid tumors may be associated with the development of aPL antibodies, enhancing the thrombophilic risk in these patients. In general, routine anticoagulant prophylaxis with low-molecular-weight heparin in cancer patients with positive aPL antibodies is strongly indicated.