Background: Cytomegalovirus (CMV) antigens are present in > 90% of GBMs but not in normal brain making it an attractive immunological target. Methods: Highly functional autologous polyclonal CMV pp65 specific T cells were expanded under GMP-compliant conditions from GBM patients and administered after 3 weeks of lymphodepleting dose-dense temozolomide (ddTMZ, 100 mg/m²). The phase I component used a 3+3 design ascending through four dose levels (5 x 10⁶ cells to 1 x 10⁸ cells). Treatment was repeated every 6 weeks for a total of 4 cycles. Dose expansion was conducted in recurrent GBM patients undergoing resection and in newly diagnosed GBM patients following concurrent chemoradiation. In vivo persistence and effector function of the adoptively transferred CMV-specific T cells was determined by dextramer staining and multiparameter flow cytometry in serially-sampled peripheral blood and in the tumor microenvironment. Results: 65 patients were screened, 25 underwent leukapheresis, and 20 completed at least 1 cycle. Median age 48 (27-69), 35% were MGMT methylated, and 10% were IDH mutated. No dose limiting toxicities (DLTs) observed. Complete radiographic response was observed in 1 patient, partial responses in 2, stable disease in 9, and progressive disease in 8. The median PFS time was 1.3 months (95% CI: 0-8.3 months) and the median OS time was 12 months (95% CI: 6 months to not reached). Repeated infusions of CMV-TC were associated with significant increase in circulating CMV+ CD8+ T cells, but cytokine production reflective of effector activity (CD107a, TNFα, IFNγ, IL2) was suppressed in these cells including directly from the GBM microenvironment. Conclusions: Adoptive infusion of CMV-specific T cells after lymphodepleting ddTMZ was well-tolerated. However, effector function of the adoptively transferred T cells was attenuated indicating further modulation of the T cell is required to prevent its dysfunction prior to proceeding to large scale clinical studies. Clinical trial information: NCT02661282.