Blood tests to detect circulating tumor cells (CTC) offer great potential to monitor disease status, gauge prognosis, and guide treatment decisions for patients with cancer. For patients with brain tumors, such as aggressive glioblastoma multiforme, CTC assays are needed that do not rely on expression of cancer cell surface biomarkers like epithelial cell adhesion molecules that brain tumors tend to lack. Here, we describe a strategy to detect CTC based on telomerase activity, which is elevated in nearly all tumor cells but not normal cells. This strategy uses an adenoviral detection system that is shown to successfully detect CTC in patients with brain tumors. Clinical data suggest that this assay might assist interpretation of treatment response in patients receiving radiotherapy, for example, to differentiate pseudoprogression from true tumor progression. These results support further development of this assay as a generalized method to detect CTC in patients with cancer.