OBJECTIVE The aim of this prospective longitudinal study was to identify static and dynamic O-(2-[(18)F]fluoroethyl)-l-tyrosine PET ((18)FET-PET)-derived imaging biomarkers in patients with glioblastoma (GBM). METHODS Seventy-nine patients with newly diagnosed GBM were included; 42 patients underwent stereotactic biopsy (unresectable tumors) and 37 patients microsurgical tumor resection. All patients were scheduled to receive radiotherapy plus concomitant and adjuvant temozolomide (RCx/TMZ). (18)FET-PET evaluation using static and dynamic analysis was done before biopsy/resection, after resection, 4 to 6 weeks following RCx, and after 3 cycles of TMZ. Endpoints were survival and progression-free-survival. Prognostic factors were obtained from proportional hazards models. RESULTS Biological tumor volume before RCx (BTVpreRCx) was the most important (18)FET-PET-derived imaging biomarker and was independent of MGMT promoter methylation and clinical prognostic factors: patients with smaller BTVpreRCx had significantly longer progression-free and overall survival (OS). (18)FET time-activity curves (TACs) before treatment and their changes after RCx were also related to outcome; patients with initially increasing TACs experienced longer OS. CONCLUSION BTVpreRCx and TAC represent important (18)FET-PET-derived imaging biomarkers in GBM. Increasing TACs are associated with prolonged OS. The BTVpreRCx is a strong prognostic factor for progression-free survival and OS independent of the mode of surgery. Our data furthermore suggest that patients harboring resectable GBM might benefit from maximal PET-guided tumor resection.