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Purpose During glutaminolysis, glutamine is catabolized to glutamate and incorporated into citric acid cycle and lipogenesis. Serum glutamate levels were measured in patients with primary prostate cancer (PCa) or metastatic castrate-resistant PCa (mCRPCa) to establish clinical relevance. The effect of glutamate-deprivation or blockade by metabotropic glutamate receptor 1 (GRM1)-antagonists was investigated on PCa cells’ growth, migration, and invasion to establish biological relevance. Experimental Design Serum glutamate levels were measured in normal men (n = 60) and patients with primary PCa (n = 197) or mCRPCa (n = 109). GRM1 expression in prostatic tissues was examined using immunohistochemistry (IHC). Cell growth, migration, and invasion were determined using cell cytotoxicity and modified Boyden chamber assays, respectively. Apoptosis was detected using immunoblotting against cleaved caspases, PARP and γ-H2AX. Results Univariate and multivariate analyses demonstrated significantly higher serum glutamate levels in Gleason score ≥ 8 than in the Gleason sscore ≤ 7 and in African Americans than in the Caucasian Americans. African Americans with mCRPCa significantly higher serum glutamate levels than those with primary PCa or benign prostate. However, in Caucasian Americans, serum glutamate levels were similar in normal research subjects and patients with mCRPC. IHC demonstrated weak or no expression of GRM1 in luminal acinar epithelial cells of normal or hyperplastic glands, but high expression in primary or metastatic PCa tissues. Glutamate deprivation or blockade decreased PCa cells’ proliferation, migration, and invasion and led to apoptotic cell death. Conclusions Glutamate expression is mechanistically associated with and may provide a biomarker of PCa aggressiveness.