Some gliomas are not noticeably enhanced on initial computer tomography (CT) scans. Such low-density non-enhancing gliomas have a relatively long period between onset and surgery ranging from 5 to 25 months. The mechanism causing the low density over a certain period of time in the sequential CT findings was retrospectively investigated. Characteristic histological findings associated with such low-density areas were microcystic formation, necrosis, intratumoral edema, infiltrative growth, and absence of capillary proliferation. Proliferation of capillary vessels is characteristic of CT enhancement.