Objective:To observe the effects of preemptive analgesia with parecoxib sodium on differentiation of peripheral T helper cells(Th) in patients with radical mastectomy .Methods :A toal of 50 patients with radical mastectomy were randomly di-vided into parecoxib group(Group P) and the control group(Group C) .Parecoxib sodium(40 mg ,diluted with normal saline into 5 mL ,the same below ) was intravenously injected 1 h before operation in Group P ,and 12 h after the first injection .The same dose of normal saline was intravenously injected in Group C at the same time-point .Patient-controlled intravenous analgesia (PCIA) with fentanyl had implemented in two groups after operation .The levels of interferon-γ(IFN-γ) ,interleukin-4(IL-4) in Th1 and Th2 were measured by flow cytometry before anesthesia(T0) ,2 h(T1) ,12 h(T2) ,24 h(T3) ,48 h(T4)after operation in the two groups .Th1 cell was marked by CD3+ CD8- IFN-γ+ .Th2 cell was marked by CD3+ CD8- IL-4+ .Th1/Th2 was calculated by the level of IFN-γ+ /IL-4+ .The visual analogue scale (VAS) ,Bruggemann comfort scale(BCS) ,dosage of fenta-nyl at each time-point in two groups were compared .Results:VAS scores in group P were significantly lower than those in Group C at T2 ,T3 ,T4 .BCS scores at T1 ,T2 ,T3 in group P were significantly higher than those in Group C .The dosages of fentany at T2 ,T3 in Group P were significantly lower than those in Group C .Compared with T0 ,Th1/Th2 ratios in the two groups at T2 ,T3 ,T4 were lower(P<0 .05) .Th1/Th2 in Group P was higher than those in Group C at T2 ,T3 ,T4(P<0 .05) . Conclusions :Th1/Th2 in patients decreases after radical mastectomy .Preemptive analgesia with parecoxib sodium assisted with fentanyl PCIA can inhibit the shift of the balance of Th1/Th2 towards Th2 type .