Anti-phosphatidylserine (APS) and anticardiolipin (ACL) antibodies were measured in 30 patients with lupus anticoagulant. In 17 cases there were clinical features of thrombosis (56.7%). The number of patients with IgG APS and ACL levels higher than seven standard deviations from the normal mean was significantly higher in the group with thrombosis (p less than 0.017 and p less than 0.02, respectively). The IgG APS level was higher in the group without systemic lupus erythematosus (p less than 0.029). There was a positive correlation between IgG APS and the activated thromboplastin time (r = 0.654, p less than 0.001). The correlation between IgG APS and IgG ACL was rho = 0.61. Two of the 5 patients with increased IgG APS and normal IgG ACL had thrombosis. We think that, in this group of patients, IgG APS titer may be, like IgG ACL, a biological marker of thrombosis risk.