Glucose and glutamine are the most abundant nutrients for producing energy and building blocks in normal and tumor cells. Increased glycolysis in tumors, the Warburg Effect, is the basis for ¹⁸F-FDG PET imaging. Cancer cells can also be genetically reprogrammed to use glutamine. 5-¹¹C-(2S)-glutamine and ¹⁸F-(2S,4R)4-fluoroglutamine may be useful complementary tools to measure changes in tumor metabolism. In glioma patients, the tracer ¹⁸F-(2S,4R)4-fluoroglutamine showed tumor-to-background contrast different from that of ¹⁸F-FDG and differences in uptake in glioma patients with clinical progression of disease versus stable disease (tumor-to-brain ratio > 3.7 in clinically active glioma tumors, minimal or no specific uptake in clinically stable tumors). These preliminary results suggest that ¹⁸F-(2S,4R)4-fluoroglutamine PET may be a new tool for probing in vivo metabolism of glutamine in cancer patients and for guiding glutamine-targeted therapeutics. Further studies of uptake mechanism, and comparison of kinetics for ¹⁸F-(2S,4R)4-fluoroglutamine versus the ¹¹C-labeled native glutamine, will be important and enlightening.