Objective To explore the relationship between the protein expression of PCNA and biological behavior science in benignant pituita ry adenoma.Methods The protein expression of PCNA in 58 patien ts with pituitary adenoma were determined by ABC immunohistochemical method.Results The PCNA index was significantly higher in the patients with recurrent pituitary adenomas than in nonrecurrent ones(P<0.05).There was no significantly difference between bleeding and unbleeding group,cystic and noncystic group,large type and unlarge type group(P>0.05 respectively) .Conclusion The protein expressions of PCNA reflected the proli ferative activties of pituitary adenomas, and could be taken as one of the indic ators to evaluate recurrence and prognosis of the tumor.