INTRODUCTION: Meningiomas account for 20% of intracranial tumors and most of them are benign. Meningiomas are histologically classified into 3 different World Health Organization (WHO) grades: grade 1 (benign), grade 2 (atypical), grade3 (anaplastic). Atypical and anaplastic meningiomas are defined by various histological criteria in hematoxylin and eosin (H & E) staining. However, these histologic criteria are sometimes insufficient and other criteria are necessary. Ki-67 (also known as MIB-1) is a monoclonal antibody staining agent, which is routinely used as a marker of cellular proliferation. METHODS: We studied and analyzed the immunohistochemical expression of Ki-67 protein in 70 cases of meningioma, including 56 benign, 5 atypical, and 9 anaplastic meningiomas. RESULTS: According to the percentage of MIB-1 positively stained cells, meningiomas were divided into 3 groups: S1% (n=49), 1.1-9.9% (n=13) and >=10% (n=8). We found the MIB-1labeling index (LI) less than 1% in 49 (87.5%) grade 1 meningiomas. On the other hand, 7 (77.7%) grade 3 meningiomas presented a MIB-1 LI more than 10%. Most of the benign meningiomas express low MIB-1 LI (mean: 1.5%). In contrast, the anaplastic types had a high MIB-1 LI (mean: 14.2%). The atypical meningiomas had MIB-1 LI in the range between benign and anaplastic types (mean: 8.6%). These differences were statistically significant among benign, atypical, and anaplastic meningiomas (P<0.001). DISCUSSION: The immunohistochemistry of Ki-67 protein (MIB-1) is valuable in determining the grade of meningiomas in histologically borderline cases.