Objective To investigate the prophylactic effect of thymosin α_1 on acute exacerbation of chronic obstructive pulmonary disease(AECOPD)in elderly patients.Methods One hundred and twenty COPD patients were randomized into two groups(n=60 for each).Patients in the treatment group received thymosin α_1 1.6 mg subcutaneously,2/week for 8 weeks;while patients in control group received normal saline instead.Serum CD3,CD4,CD8,IrA,IgG and IgM levers were measured,and the pulmonary function were examined before and 2.12 months after treatment.Results In treatment group the recurrence interval of AECOPD Was extended:the attack frequency and persistence time of AECOPD were significantly lower in cutuparison with those of the control group(P＜0.05).CD4 and CD4/CD8 levels were significanfly increased(P＜0.05)after treatment of thymosincd.Conclusion Thymosin α_1 Can protect acute exacerbation of chronic obstructive pulmonary disease in elderly patients.