INTRODUCTION The unpredictable biology of pituitary adenomas makes it a therapeutic challenge. Moreover ,histopathology of pituitary carcinomas and locally invasive adenomas are indistinguishable from benign tumors and a new marker which would enable to differentiate those lesions is vital. The aim of the study was to evaluate Ki-67 and PTTG (pituitary tumour--transforming gene) expression in pituitary adenomas and their applicationas markers of tumour aggressiveness. MATERIAL AND METHODS A retrospective analysis of 55 patients: 32 females(58%) and 23 males (42%), mean age 50 ± 16 years who underwent pituitary tumor surgery between 2003-2012. Ki-67 and PTTG indices were determined by immunohistochemical staining. Magnetic resonance imaging or computed tomography was performed beforehand and one year after surgery to figure a potential tumour progression, tumour size and correlation to adjacent tissues. RESULTS The expression of Ki-67and PTTG was revealed in cell nucleiin 88% and 85% of adenomas, respectively. The median Ki-67 and PTTG indices were 1.4 and 1.0, respectively(p = 0.006). In the group with macroadenoma as compared with the group with microadenoma, median Ki-67 index was higher (1.4% vs. 1.03%; p = 0.02). We did not find correlation between both Ki-67 and PTTG indices and tumour progression. Tumours with positive immunostaining towards FSH revealed lower Ki-67 and PTTG indices than the rest with a negative one (0.6% vs.1.84%, p = 0.0004 and 0.67% vs 1.23%,p = 0.047; respectively). However, PTTG index was higher in the group with acromegaly as compared to the group with clinically non-functioning pituitary adenoma (NFPA) (1.28% vs.0.35%; p = 0.02). CONCLUSIONS Positive nuclear expression of Ki-67 and PTTG was observed in the majority of pituitary adenomas. Only higher Ki-67 expression was related to the tumour invasiveness found on MRI/CT. Tumour progressionwas not related to both Ki-67 and PTTG expression.