BACKGROUND Tenascin-C (TNC) has been reported to be a useful biomarker for the activity of inflammatory diseases. This study investigated the association between TNC levels in the cerebrospinal fluid (CSF) and symptomatic vasospasm after aneurysmal subarachnoid hemorrhage (SAH), and the prognostic value of TNC levels. METHODS TNC levels were measured in CSF in 33 consecutive patients diagnosed with aneurysmal SAH of Fisher computed tomography group III and were compared between those with and without subsequent cerebral vasospasm. Factors influencing symptomatic vasospasm were determined using multivariate logistic regression analyses. The receiver-operating characteristic curve technique was used to assess specificity and sensitivity in the prediction of symptomatic vasospasm. RESULTS The CSF TNC levels peaked immediately after SAH and were significantly higher in patients who subsequently developed symptomatic vasospasm than in those who did not. On multivariate analyses, higher TNC levels in the CSF (odds ratio, 1.059; 95% confidence interval, 1.023-1.096; P<0.001) and World Federation of Neurosurgical Societies grades IV to V on admission (odds ratio, 3.238; 95% confidence interval, 1.033-10.152; P<0.05) significantly predicted symptomatic vasospasm. To predict the onset of symptomatic vasospasm, 16.2 ng/mL was considered as an appropriate cut-off value for CSF TNC on days 1 through 6, giving a sensitivity of 81.0% and a specificity of 79.5% (negative and positive predictive values: 82.3% and 76.7%, respectively). CONCLUSIONS TNC in the CSF may be a useful biomarker for predicting subsequent development of cerebral vasospasm.