Inactivation of mismatch repair (MMR) genes may predict sensitivity to immunotherapy in metastatic prostate cancers. We studied primary prostate tumors with MMR defects. A total of 1,133 primary prostatic adenocarcinomas and 43 prostatic small cell carcinomas (NEPC) were screened by MSH2 immunohistochemistry with confirmation by next-generation sequencing (NGS). Microsatellite instability (MSI) was assessed by PCR and NGS (mSINGS). Of primary adenocarcinomas and NEPC, 1.2% (14/1,176) had MSH2 loss. Overall, 8% (7/91) of adenocarcinomas with primary Gleason pattern 5 (Gleason score 9-10) had MSH2 loss compared with 0.4% (5/1,042) of tumors with any other scores ( < 0.05). Five percent (2/43) of NEPC had MSH2 loss. MSH2 was generally homogenously lost, suggesting it was an early/clonal event. NGS confirmed loss-of-function alterations in all (12/12) samples, with biallelic inactivation in 83% (10/12) and hypermutation in 83% (10/12). Overall, 61% (8/13) and 58% (7/12) of patients had definite MSI by PCR and mSINGS, respectively. Three patients (25%) had germline mutations in Tumors with MSH2 loss had a higher density of infiltrating CD8 lymphocytes compared with grade-matched controls without MSH2 loss (390 vs. 76 cells/mm ; = 0.008), and CD8 density was correlated with mutation burden among cases with MSH2 loss ( = 0.72, = 0.005). T-cell receptor sequencing on a subset revealed a trend toward higher clonality in cases versus controls. Loss of MSH2 protein is correlated with inactivation, hypermutation, and higher tumor-infiltrating lymphocyte density, and appears most common among very high-grade primary tumors, for which routine screening may be warranted if validated in additional cohorts. .