Purpose: Positron emission tomography (PET) with [18F] fluorodeoxyglucose (FDG-PET) has increasingly been used to evaluate the efficacy of anticancer agents. We investigated the role of FDG-PET as a predictive marker for response to mammalian target of rapamycin (mTOR) inhibition in advanced solid tumor patients and in murine xenograft models. Patients and Methods: Thirty-four rapamycin-treated patients with assessable baseline and treatment FDG-PET and computed tomography scans were analyzed from two clinical trials. Clinical response was evaluated according to Response Evaluation Criteria in Solid Tumors, and FDG-PET response was evaluated by quantitative changes and European Organisation for Research and Treatment of Cancer (EORTC) criteria. Six murine xenograft tumor models were treated with temsirolimus. Small animal FDG-PET scans were performed at baseline and during treatment. The tumors were analyzed for the expression of pAkt and GLUT1. Results: Fifty percent of patients with increased FDG-PET uptake and 46% with decreased uptake had progressive disease (PD). No objective response was observed. By EORTC criteria, the sensitivity of progressive metabolic disease on FDG-PET in predicting PD was 19%. Preclinical studies demonstrated similar findings, and FDG-PET response correlated with pAkt activation and plasma membrane GLUT1 expression. Conclusion: FDG-PET is not predictive of proliferative response to mTOR inhibitor therapy in both clinical and preclinical studies. Our findings suggest that mTOR inhibitors suppress the formation of mTORC2 complex, resulting in the inhibition of Akt and glycolysis independent of proliferation in a subset of tumors. Changes in FDG-PET may be a pharmacodynamic marker for Akt activation during mTOR inhibitor therapy. FDG-PET may be used to identify patients with persistent Akt activation following mTOR inhibitor therapy.