Abstract While tumors of the nervous system (NS) are rare, they contribute to significant morbidity and mortality. Despite advances in imaging technology, initial diagnosis and characterization of treatment response remains problematic for devastating tumors such as glioblastoma (GBM). A liquid biopsy can provide a minimally-invasive and serial source of biomarkers to supplement current diagnostic methods. Here, we utilize a unique characteristic of NS tumors including GBM and schwannoma/neurofibroma – the overexpression of the a2 variant of the IL-13 receptor which is not detectable in normal tissue. Previously, we have shown that IL13Ra2 is elevated in the plasma of some GBM patients above baseline levels. We hypothesize that the presence of circulating IL13Ra2 not only predicts the level of this receptor in the tumor tissue, but also has clinical prognostic value. Here, we used ELISA to measure the plasma levels of IL13Ra2 in the GBM (n=77) and schwannoma/neurofibroma (n=15) patients and correlated these levels to tumor levels, diagnosis, and clinical characteristics. We have demonstrated that elevated levels of plasma IL13Ra2 correlated with tumor presence compared to non-tumor controls for both GBM and schwannoma/neurofibroma patients, indicating this biomarker’s diagnostic utility. Further, for both GBM and schwannoma/neurofibroma patients the plasma concentration of IL13Ra2 is correlated to its tumor concentration, indicating that sampling patient plasma could provide insight into the tumor’s composition. Finally, elevated levels of IL13Ra2 protein in the tumor and plasma of GBM patients predicted longer patient survival. These exciting findings lay the groundwork for a plasma-based NS tumor liquid biopsy.