Tenascin, an extracellular matrix glycoprotein, has been reported to be expressed predominantly on glioma tissue in the CNS, both in a cell associated and an excreted form. Recently, a highly sensitive sandwich type enzyme immunoassay for quantitative determination of tenascin was developed. In the present study, the amount of tenascin in CSF was measured. An increase of tenascin in CSF (> 100 ng/ml) was found in patients with an astrocytic tumour. The concentration was significantly higher (> 300 ng/ml) in high grade astrocytoma (anaplastic astrocytoma and glioblastoma) and a further increase (> 1000 ng/ml) was found in cases of CSF dissemination of high grade astrocytoma. On the other hand, tenascin concentrations were less than 100 ng/ml in non-astrocytic tumours and non-neoplastic neurological diseases, except meningeal dissemination of tumour cells, meningeal stimulation by infection, and subarachnoid haemorrhage. In cases of treated astrocytomas in remission, tenascin was negligible (< 100 ng/ml) in the CSF. The measurement of tenascin in CSF is useful for differential diagnosis of brain tumours and monitoring of astrocytic tumours.