Aim To investigate changes of the level of serum soluble CD40 ligand (sCD40L) in the patients with acute coronary syndrome (ACS) so as to know if sCD40L can be as a serum marker of plaque vulnerability,and explore the relationship of sCD40L with the traditional risk factors of coronary heart disease (CHD) and its clinical value in early diagnosis of ACS. Methods All the cases of CHD and non-CHD were divided into acute myocardial infarction (AMI) group (n=34),unstable angina (UA) group (n=31) ,stable angina (SA) group (n=30) and non-coronary artery disease (NCAD) group (n=40). In all these groups,the level of sCD40L was detected by the enzyme-linked immuneosorbent assay (ELISA). Results (1)The levels of sCD40L were higher in AMI group (8.02±4.03 μg/L) and in UA (8.35 ± 3.89 μg/L) than those of SA group (4.86±2.23 μg/L) and NCAD group (4.35±1.83 μg/L) (all P<0.01); the difference of sCD40L levels between SA group and NCAD group,and between UA group and AMI group,were not significant (all P>0.05). (2)The levels of sCD40L in men of four groups were higher than those of women,but the difference between men and women was no statisticly significant (P>0.05); The level of sCD40L was not correlated with body mass index,age,smoking status,hypertension,diabetes mellitus,family history of CHD (P>0.05); The level of sCD40L in patients with CHD was positively correlated with lipoproteina (Lp(a)) (r=0.567,P=0.001),negatively correlated with high density lipoprotein cholesterol (HDLC) (r=-0.365,P=0.0001). (3) In diagnosis of ACS,it was found that the area under ROC curve of sCD40L was 0.872,the sensitivity,specificity and validity were 82.8%,61.9 % and 85.7% respectively,which was the highest among all indexes including sCD40L and LP (a) and creatine kinase-MB (CK-MB). The best critical level of sCD40L was 4.58 μg/L. Conclusion The levels of sCD40L may reflect the vulnerability of atherosclerotic plaque. sCD40L is an independent risk factor in CHD. In diagnosis of ACS ,sCD40L has higher sensitivity,specificity and validity.