The clinical status of patients with glioma is influenced by 1) the histological malignancy of the tumor, 2) the tumor volume, 3) secondary status such as brain edema or intracranial hypertension due to the tumor, and 4) the host immunity. Due to some improvement in at least 2) and 3) by the initial treatment, most low grade glioma cases pass through a clinically silent postsurgical period. However, at a certain point, transition to a high grade tumor malignant transformation may occur with exacerbation of the symptoms. Twenty-two cases of histologically established low grade glioma experienced over the past 7 years, in which immunological status was evaluated, were analyzed. Nine cases (41%) showed malignant transformation. Characteristic pictures of the clinical symptoms, computed tomography (CT) scan findings, immunological status, and morphological findings (mainly immunohistochemical examination) in nine cases were delineated. The findings at the time of exacerbation of the symptoms were as follows. In all cases CT scan demonstrated the change in the main lesion from low density to mixed density and were compatible with a high grade glioma. Reduction in host immunity was verified. Morphological increase in the tumor volume, increase in histological malignancy and deterioration in the secondary status due to the tumor were confirmed. Necrosis of the tumor cells as well as increase in giant cells and gemistocytes were observed. Immunohistochemical analysis revealed a decrease and irregularity in glial fibrillary acidic protein positive cells and positive processes as well as increase in vimentin intensity. These findings demonstrate change in the biological characteristics of the tumor.(ABSTRACT TRUNCATED AT 250 WORDS)