Transforming growth factor-beta (TGF-beta) signaling is involved in glioma development. The monoclonal antibody fresolimumab (GC1008) can neutralize all mammalian isoforms of TGF-beta, and tumor uptake can be visualized and quantified with Zr-89-fresolimumab PET in mice. The aim of this study was to investigate the fresolimumab uptake in recurrent high-grade gliomas using Zr-89-fresolimumab PET and to assess treatment outcome in patients with recurrent high-grade glioma treated with fresolimumab. Methods: Patients with recurrent glioma were eligible. After intravenous administration of 37 MBq (5 mg) of Zr-89-fresolimumab, PET scans were acquired on day 2 or day 4 after tracer injection. Thereafter, patients were treated with 5 mg of fresolimumab per kilogram intravenously every 3 wk. Zr-89-fresolimumab tumor uptake was quantified as maximum standardized uptake value (SUVmax). MR imaging for response evaluation was performed after 3 infusions or as clinically indicated. Results: Twelve patients with recurrent high-grade glioma were included: 10 glioblastomas, 1 anaplastic oligodendroglioma, and 1 anaplastic astrocytoma. All patients underwent Zr-89-fresolimumab PET 4 d after injection. In 4 patients, an additional PET scan was obtained on day 2 after injection. SUVmax on day 4 in tumor lesions was 4.6 (range, 1.5-13.9) versus a median SUVmean of 0.3 (range, 0.2-0.5) in normal brain tissue. All patients showed clinical or radiologic progression after 1-3 infusions of fresolimumab. Median progression-free survival was 61 d (range, 25-80 d), and median overall survival was 106 d (range, 37-417 d). Conclusion: Zr-89-fresolimumab penetrated recurrent high-grade gliomas very well but did not result in clinical benefit.