Objective To investigate the protection effects of cimetidine for the immune function of patients underwent cardiac operation under cardiopulmonary bypass (CPB). Methods From Jan. 2004 to Jan. 2005, thirty patients suffered from rheumatic cardiac valvular disease received cardiac valve replacement in our hospital, and were divided into cimetidine group and control group.The effects of cimetidine on cellular immune, fluid immune and erythrocytic immune were observed 1d before operation, 1, 3, 5, 7 and 14d after operation. Results After operation,CD3,CD4,CD4/CD8, NK cell activity, Interleukin-2(IL-2), RBC-C3bRR and RBC-ICR in cimetidine group were significantly higher than those in the control group(Plt;0.01). In cimetidine group,those index began to recover on the postoperative 3 to 5 days, and return to normal level on the postoperative 7 days (Pgt;0.05). In control group, 7 and 14 days respectively. Conclusion The protective effects of cimetidine on immune function of openheart operative patients are significant.