The pathological effects of thrombospondin-1 (TSP-1) have been studied in many preclinical tumor models and rheumatoid arthritis. However, the role of TSP-1 and anti-thrombospondin-1 autoantibodies (ATSA) in systemic lupus erythematosus (SLE) has not been specifically defined. In this study, we investigated the clinical relevance and functional significance of TSP-1 and ATSA in SLE patients. Serum levels of TSP-1 and ATSA were measured by ELISA in 138 adult SLE patients and 60 healthy controls. Blood cell counts, rheumatoid factor (RF), immunoglobulins, erythrocyte sedimentation rate (ESR), complements, and SLE-related autoantibodies were measured by standard laboratory techniques. Disease activity was assessed by systemic lupus erythematosus disease activity index (SLEDAI). TSP-1 concentrations were significantly lower in SLE patients compared with those in healthy controls. A significant difference of TSP-1 was observed in the patients with serositis, C3 decrease, RF positive, leukocytopenia, and thrombocytopenia. The levels of TSP-1 showed a positive correlation with the number of leukocyte and thrombocyte, while a negative correlation with anti-dsDNA antibody, IgG, ESR, and SLEDAI. ATSA was observed in 58.7% (81/138) of SLE patients, which was significantly higher than that in healthy controls (7/60, p?