The causes of thrombosis and pregnancy loss in antiphospholipid syndrome (APS) are still unknown, although several hypotheses have been proposed and hypofibrinolysis has been implicated. Anti-tissue-type plasminogen activator (tPA) antibodies may induce fibrinolytic defects and preliminary data indicate an association with thrombosis in APS. We measured plasma anti-tPA antibody levels in 91 consecutive patients with APS, 91 healthy controls, 40 patients with antiphospholipid antibodies without APS symptoms, and 23 patients with systemic lupus erythematosus (SLE) without antiphospholipid antibodies and APS symptoms. Patients with APS had anti-tPA antibody levels higher than controls (P = .0001), patients with SLE (P = .0001), and asymptomatic antiphospholipid patients (P = .05). A subgroup of 53 patients had plasma levels of tPA antigen higher (P = .0001) and tPA activity lower (P = .05) than controls, with an inverse correlation (r = -0.454; P = .003) between anti-tPA antibody levels and tPA activity and no correlation with tPA antigen. The 2 patients with the highest antibody levels had tPA activity below the normal range. Their antibodies were, respectively, IgG1 and IgG3; both recognized human tPA, recombinant tPA, and the catalytic domain of tPA, but not beta 2-glycoprotein I, prothrombin, or plasminogen. Our data indicate that anti-tPA antibodies specifically interacting with the catalytic domain of tPA can be found in patients with APS, representing a possible cause of hypofibrinolysis.