The objective of the present study was an evaluation of the incidence and risk factors for erroneous histopathological diagnosis of low-grade glioma after stereotactic biopsy. Twenty-eight tumors diagnosed as low-grade glioma after stereotactic biopsy and surgically resected thereafter were analyzed. There were 13 astrocytomas, 7 oligodendrogliomas, and 8 mixed gliomas. All neoplasms had a lobar location. Seven tumors had contrast enhancement on MRI. The number of tissue samples obtained during stereotactic biopsy was one in 19 cases, two in 4, and three or more in 5. Complete diagnostic agreement in tumor typing and grading after stereotactic biopsy and surgical resection was attained in 10 cases (36%). Agreement in tumor typing was marked in 16 cases (57%). Erroneous typing was more frequent in tumors with an MIB-1 index of less than 3% (P = 0.0629) and mixed gliomas (P = 0.0801). Overgrading of WHO grade I tumors was marked in 3 cases (11%) and undergrading of WHO grade III gliomas in 8 cases (28%). Tumor undergrading was more frequent in cases with an MIB-1 index of more than 3% (P = 0.0045). The MIB-1 index detected after stereotactic biopsy was nearly always lower compared with those established after surgical resection (P < 0.0001). In conclusion, the histopathological diagnosis of low-grade glioma established after stereotactic biopsy is associated with a substantial risk of inaccuracy. Tumors with low proliferative activity and mixed gliomas are especially susceptible for erroneous tumor typing. Undergrading of high-grade gliomas may be suspected if the MIB-1 index in the tumor specimen constitutes more, than 3%.