Monitoring of radiochemotherapy (RCX) in patients with glioblastoma is difficult because unspecific alterations in magnetic resonance imaging with contrast enhancement can mimic tumor progression. Changes in tumor to brain ratios (TBRs) in positron emission tomography (PET) usingO-(2-[18F]fluoroethyl)-l-tyrosine (18F-FET) after RCX with temozolomide of patients with glioblastoma have been shown to be valuable parameters to predict survival. The kinetic behavior of 18F-FET in the tumors is another promising parameter to analyze tumor metabolism. In this study, we investigated the predictive value of dynamic 18F-FET PET during RCX of glioblastoma. Time-activity curves (TACs) of 18F-FET uptake of 25 patients with glioblastoma were evaluated after surgery (FET-1), early (7–10 days) after completion of RCX (FET-2), and 6 to 8 weeks later (FET-3). Changes in the time to peak (TTP) and the slope of the TAC (10–50 minutes postinjection) were analyzed and related to survival. Changes in kinetic parameters of 18F-FET uptake after RCX showed no relationship with survival time. In contrast, the high predictive value of changes of TBR to predict survival was confirmed. We conclude that dynamic 18F-FET PET does not provide additional prognostic information during RCX. Static 18F-FET PET imaging (20–40 minutes postinjection) appears to be sufficient for this purpose and reduces costs.