Activation of the phosphoinositide 3-kinase signaling cascade, often through loss of the phosphatase and tensin homologue deleted on chromosome 10 (PTEN) tumor suppressor, is frequent in castration-resistant prostate cancer (CRPC). We assessed the safety and efficacy of combining the mammalian target of rapamycin (mTOR) inhibitor everolimus with docetaxel in a phase I clinical trial of men with metastatic CRPC, and evaluated the ability of fluorine–18-fluorodeoxyglucose (FDG) positron emission tomography (PET) to predict response to treatment. The observed clinical activity of tolerable dose levels of everolimus with docetaxel was low. FDG-PET might serve as a biomarker for target inhibition by mTOR inhibitors in metastatic CRPC.