Summary. In a first study, we performed a cross-sectional analysis of urinary excretion of isoprostanes, IPF2α-III and VI, and monocyte tissue factor (TF) antigen and activity between 11 antiphospholipid (APL) antibody-positive patients and 13 APL negative subjects. In a second study, 11 APL positive patients were randomly supplemented either with (n = 6) or without (n = 5) antioxidants (vitamin E at 900 IU day−1, vitamin C at 2000 mg day−1) for 6 weeks. In a third study, TF and superoxide anion were measured in human monocytes incubated with anti-β2 glycoprotein 1 (β2GP1) or control IgG, either with or without vitamin E. APL-positive patients had higher values of isoprostanes (P < 0.05) and monocyte TF antigen (P = 0.001) and activity (P = 0.0001) than APL-negative subjects. Only in APL positive patients did monocyte TF antigen correlate significantly with IPF2α-III (rho 0.79; P < 0.003) and IPF2α-VI (rho = 0.87; P < 0.0001). In patients who received antioxidant supplementation, we found a significant decrease of isoprostanes (P < 0.05) and monocyte TF antigen (P < 0.01) and activity (P < 0.007). In vitro experiments demonstrated that anti-β2GP1 antibodies dose-dependently enhanced the monocyte production of the superoxide anion and TF, which were significantly inhibited by vitamin E. This study demonstrates that in APL-positive patients, oxidative stress contributes to activate the clotting system via over-expression of monocyte TF. We suggest that anti-β2GP1 antibodies could play a pivotal role by enhancing the monocyte production of oxygen free radicals.