Abstract: Objective To determine the expression of O6-methylguanine-DNA methyltransferase (MGMT) in pituitary adenomas and its association with tumour aggressiveness. Methods From October 2011 to August 2012, all the patients with pathologically confirmed pituitary adenomas in our hospital were recruited for this study. The expression of MGMT in the tumor tissues was detected with immunohistochemistry. Results A total of 138 (including 71 male) patients participated in this study. The majority of the participants had nonfunctional (71%) and macroadenomas (92.8%). Aggressive and relapsed pituitary adenomas accounted for 31.2% and 8.7% of the cases, respectively. Low MGMT expression was found in 79 cases. The MGMT expression had no significant associations with gender, age, aggressiveness and relapse of tumors. However, prolactinomas were more likely to have low MGMT expression than nonfunctional adenomas (P=0.025) and growth hormone adenomas (P=0.043). Conclusion The expression of MGMT has no associations with the aggressiveness and relapse of pituitary adenomas. Prolactinomas are more likely to have low expression of MGMT, which indicates that temozolomide might become an alternative treatment for aggressive and dopamine resistant prolactinomas.