Neuron specific enolase (NSE) in the cerebrospinal fluid (CSF) and serum of 54 cases of brain tumors, in the tumor tissues of 10 brain tumors and in the cyst fluids of 7 brain tumors was measured by radioimmunoassay with NSE measurement kit (Eiken Chemistry co.) NSE values in the cerebrospinal fluid of 35 malignant brain tumors showed abnormal increase higher than 10 ng/ml in 17 cases (about 50%) of them, whereas all of benign cases were lower than 10 ng/ml. The means and standard deviations of NSE in the cerebrospinal fluid of malignant tumors were 20.63 +/- 20.78 ng/ml in the astrocytoma grade 3 and 4, 19.73 +/- 15.5 ng/ml in the medulloblastoma and 12.4 +/- 8.9 ng/ml in the germ cell tumor. NSE values in the CSF of 12 brain tumors with intrathecal metastasis were 28.0 +/- 18.9 ng/ml (mean +/- SD) showing about three times as much as those without intrathecal metastasis. There was significant difference between these groups (p less than 0.01). All cyst fluids including in 7 cases of brain tumors demonstrated high values of NSE even if they were benign tumor and their NSE values in CSF were normal. NSE values of brain tumor tissues had no correlation with malignancy, but were higher in the periphery of the tumors than in the center. NSE values of CSF in brain tumors might be changed in relation with the improvement or deterioration on clinical state and CT. It might be suggested that the measurement of NSE in the CSF had significance as a monitor of therapeutic efficacy and prognosis for the brain tumors.