Proliferative activity in 78 glioma specimens was assessed immunohistochemically by determining proliferating index of tumor cells (PTC-PI) and endothelial cells (PEC-PI) using the MIB-1 monoclonal antibody. The PTC-PI of anaplastic astrocytoma (9.0 +/- 5.8: mean + standard deviation) was significantly higher than that of astrocytoma (1.2 +/- 0.4, < 0.01), and lower than that of glioblastoma multiforme (12.0 +/- 5.6, < 0.05). We then compared PTC-PI values with the prognosis of patients with malignant glioma (both glioblastoma and anaplastic astrocytoma). Kaplan-Meier survival rate analysis demonstrated higher survival rates in patients with less than 8.0% of PTC-PI at 5 and 10 years (p < 0.05). These results suggested PTC-PI provides useful information which may allow better assessment of the biological behaviour and clinical prognosis of glioma, in addition to histological grading. While the average PEC-PI value (3.3) was lower than that of PTC-PI (7.0), there was a significantly close relationship between them (p < 0.01), fostering the developments novel therapies directed towards suppression of microvascular regeneration.