Purpose: We conducted a pilot study to assess the feasibility and the potential implications of detecting TERT promoter (TERTp)-mutant cell-free tumor-derived DNA (tDNA) in the cerebrospinal fluid (CSF) and plasma of glioblastoma patients. Experimental Design: Matched CSF and plasma samples were collected in 60 patients with glial tumors. The CSF collection was obtained during surgery, before any surgical manipulation of the tumor. The extracted tDNA and corresponding tumor DNA samples were analyzed for TERTp and isocitrate dehydrogenase (IDH) hotspot mutations. In addition, the variant allele frequency (VAF) of TERTpmutationintheCSF-tDNA was correlated with tumor features and patients' outcome. Results: Thirty-eight patients had TERTp-mutant/IDH wild-type glioblastomas. The matched TERTp mutation in the CSF-tDNA was successfully detected with 100% specificity (95% CI, 87.6-100%) and 92.1% sensitivity (95% CI, 78.6- 98.3%) (n = 35/38). In contrast, the sensitivity in the plasmatDNA was far lower [n = 3/38, 7.9% (95% CI, 1.6-21.4%)]. We concordantly observed a longer overall survival of patients with low VAF in the CSF-tDNA when compared with patients with high VAF, irrespective of using the lower quartile VAF [11.45%; 14.0 mo. (95% confidence interval, CI, 10.3-17.6) vs. 8.6 mo. (95% CI, 4.1-13.2), P = 0.035], the lower third VAF [13%; 15.4 mo. (95% CI, 11.6-19.2) vs. 8.3 mo. (95% CI, 2.3-14.4), P = 0.008], or the median VAF [20.3%; 14.0 mo. (95% CI, 9.2-18.7) vs. 8.6 mo. (95% CI, 7.5-9.8), P = 0.062] to dichotomize the patients. Conclusions: This pilot study highlights the value of CSF-tDNA for an accurate and reliable detection of TERTp mutations. Furthermore, our findings suggest that high TERTp mutation VAF levels in the CSF-tDNA may represent a suitable predictor of poor survival in glioblastoma patients. Further studies are needed to complement the findings of our exploratory analysis. Clin Cancer Res; 24(21); 5282-91.