Abstract： Objective To investigate clinical implications of expression of CD40L in monocytes and changes in serum soluble CD40L in patients with acute coronary syndromes (ACS). Methods Sixteen control and 56 patients, including 24 with stable angina (SA), 20 with unstable angina (UA) and 12 with acute myocardial infarction (AMI) enrolled in this study. Expression of CD40L in monocytes was analyzed by flow cytometry and sCD40L levels were measured by ELISA. Results Expression of CD40L in monocytes and serum levels of sCD40L in UA and AMI patients were higher than in SA patients and controls. In patients with AMI, sCD40L levels showed no significant increase when compared to patients with UA, while AMI patients had a peak level of sCD40L at 24 hours after AMI. PTCA induced a marked rise in sCD40L levels in all patients, while CD40L expression in monocytes showed no difference between patients with PTCA, before and after. Conclusion Enhanced level of serum sCD40L may be a reliable prognostic indicator for ACS and may represent a marker of coronary disease activity.