Objective, Annexins (Anx) are a family of structurally related proteins that bind to phospholipids in a calcium dependent manner. It has been reported that antibodies to Anx V, which acts as an antithrombotic protein, are associated with thrombosis in systemic lupus erythematosus (SLE) and/or antiphospholipid syndrome (APS). Homology between the primary structures of Anx IV and Anx V is the highest among members of the Anx family. We investigated whether anti-Anx IV autoantibodies can be detected in the sera of patients with SLE and/or APS. Methods. Seventy-four patients with SLE/APS were divided into 3 groups: Group A, patients with SLE but no clinical or serological features of APS; Group B, patients with SLE having only serological signs of APS; and Group C, patients with clinical symptoms and serological signs of AP, AN;IV and Anx V were prepared by recombinant technique. Anti-Anx IV, Anx V, cardiolipin (CL), and CL beta(2)-glycoprotein I were detected by ELISA. Results. Anti-Anx IV was found in 15.4% of Group A, 20.0% of Group B, and 21.7% of Group C, Anti-Anx V was found in 3.8% of Group A, 28.0% of Group B, and 30.4% of Group C. Significant correlations were noted between anti-Anx IV titer and anti-Anx V titer (p < 0.001), and between anti-Anx IV titer and aCL titer (p < 0.01). Conclusion. Anti-Anx TV and V antibodies were characterized in the sera of patients with SLE/APS. Significantly higher frequency of arterial or venous thrombosis was found in patients with anti-Anx V.