Using a fluorescein surgical microscope that we developed, we performed surgery on 30 cases of malignant glioma. Operative findings and histological findings were then compared in five of these cases. Fluorescein sodium was systemically administered intravenously as a fluorescent dye. About 20 min after intravenous administration, fluorescein activity in the blood decreased, and the fluorescence was observed only in the area lacking the blood-brain barrier function, such as tumors; then, resection of the tumor was started. The fluorescent regions coincided with the enhanced regions on CT and MRI. Fluorescein spread to the surrounding brain edematous region with time, but its intensity was very different from the tumor-associated one. In the histological examination of intensely fluorescent regions, abnormal tumor blood vessels with a thick wall and a small caliber, or with a thin wall and a large caliber, were observed. Dense tumor cells were found in these regions. On the other hand, in regions with weak or no fluorescence, infiltration of tumor cells was scant, and no abnormal blood vessels were found. Fluorescence was not observed in necrotic regions of the tumor center. These histological findings coincided with those obtained in large surgical specimen or autopsy, and tumor-cell rich regions seemed to be removed by resecting abnormal vascular regions (fluorescent regions). Enhanced regions on CT and MRI disappeared after operation by resecting intensely fluorescent regions. These results indicate that the fluorescein surgical microscope developed by us is a useful aid in operations on malignant glioma.