Aim To investigate the clinical implications of the expression of CD40 ligand (CD40L) on peripheral blood monocytes and sera soluble CD40L (sCD40L) changes in patients with unstable angina. Methods 16 normal controls and 40 patients including 20 with SA(8 patients by PTCA), 20 with UA entered in this study. The expression of CD40L on monocytes was analyzed by indirect-immonofluorescence flow cytometry and sera sCD40L levels were measured by ELISA. Results (1) The expression of CD40L on monocytes in UA were higher compared with SA and controls (P<0.01), while no significant difference was found between SA and controls(P>0.05). (2) Patients with UA had significantly raised sCD40L sera levels when compared with patients with SA and controls(P<0.01). Values also differed significantly between patients with SA and controls. (3) PTCA induced a marked rise in sCD40L levels in all patients, while the expression of CD40L on monocytes showed no significantly difference between patients with PTCA before and after. (4) A significant correlation was observed between sCD40L sera concentrations and the number of complex lesions(r=0.54,P<0.01). sCD40L, however, was not significantly associated with disease extent and range. Conclusions The enhanced level of serum soluble CD40L may play an important role in coronary plaque instability and disruption, and may represent a marker of coronary disease activity.