Objective: To detect the serum levels of soluble endothelial glycoprotein endoglin (s-Eng) in patients with antiphospholipid syndrome(APS) and to evaluate the correlation between s-Eng levels and clinical features and laboratory parameters.Methods:The levels of serum s-Eng were measured by enzyme linked immunosorbent assay ( ELISA) in 139 patients with APS, 44 patients with SLE but no APS, 37 patients with primary Sjogren’s syndrome (pSS), 23 patients with Bechet’s disease (BD), 22 patients with systemic sclerosis (SSc) and 22 persistent anticardiolipin antibody (aCL) positive indivi-duals without SLE or APS(simply aCL positive group) and 87 health controls(HC) without any auto-immune diseases. These APS patients included 64 primary APS patients and 75 APS patients secondary to SLE.The correlation between the clinical data, laboratory parameters, and serum s-Eng levels were analyzed.Independent samples t test,paired t test,Chi-square Test, Mann-Whitney U test, Pearson’s χ ² test were used for statistical analyses. Results:(1)The serum levels of s-Eng were significantly higher in the patients with APS whether primary or secondary to SLE than in the health controls and simply aCL positive group and the patients with other autoimmune diseases, including SLE, pSS, BD and SSc(P<0.001).There was no significant difference in the serum s-Eng levels between simply aCL positive group and health controls[(5.17±2.00) mg/L vs.(5.04±1.11) mg/L, P>0.05]. (2) The best cut-off value for the diagnosis of APS was no less than 8.37 mg/L as mean ± 3SD value, with the sensitivity at 0.772 and the specificity at 0.928. The Youden index was 0.700. These results indicated good validity of s-Eng as a diagnostic marker for APS. (3)The proportions of artery thrombosis and pathological pregnancy were higher in the group of s-Eng -positive APS patients than that in s-Eng -negative group(46/81 vs. 19/58,29/65 vs. 10/44,respectively, all P<0.05).The levels of PLT were lower in the group of s-Eng-positive APS patients (72.00×10 ⁹/L vs. 119.00×10 ⁹/L, P<0.001). (4)The proportions of the presence(93.83% vs. 37.93%, P<0.001) and titer (61.70 U/mL vs.15.45 U/mL, P<0.001) of aCL were both higher in the group of s-Eng-positive APS patients than in s-Eng-negative group. The proportions of the presence (61.73% vs. 43.10%, P<0.05)and titer (33.48 U/mL vs.17.40 U/mL,P<0.05) of anti-β2-glycoproteinⅠ antibody were both higher in the group of s-Eng -positive APS patients than in s-Eng -negative group too. Conclusion:S-Eng serum levels were significantly increased in the patients with APS, and it may play a role as acomplementary serological marker for the diagnosis and risk prediction of APS.