Pituitary adenomas account for 10% to 15% of intracranial neoplasms. Multiple factors had been introduced for tumor recurrence. MIB-1 monoclonal antibody, a marker of the proliferative index, has been introduced in various tumors, but unfortunately, the usefulness of MIB-1 in predicting the behavior of pituitary adenoma has been debated recently. Hence, the aim of this study was to evaluate its role in the recurrence and the invasiveness of pituitary adenomas. In a cross-sectional study conducted between 2005 and 2010, patients with a diagnosis of pituitary adenoma, referred to 2 medical centers (Arad and Iran Mehr hospital) in Tehran, were enrolled. Clinical and laboratory data were gathered. Immunostaining for MIB-1 monoclonal antibody was performed for each slide and scored blindly by 2 pathologists. A total of 176 surgically treated patients met the inclusion criteria. Thirty-six of 176 cases had recurrent episodes. Eleven of 176 patients had invasive tumors. The mean Ki67 index was 2.7%±3.96% in recurrent cases and 2.24%±2.35% in nonrecurring cases, with no statistically significant difference between them (P>0.05). Ten of 11 invasive pituitary tumors had a Ki67>3%, and a statistically significant difference was observed between invasive and noninvasive tumors (P<0.05). Hence, the Ki67 index seems to be suitable for predicting invasion, but it failed to show any correlation between Ki67 and tumor recurrence.