Purpose: To evaluate the possible mechanisms influencing the infiltration of CD8 T lymphocytes into the tumor epithelium of advanced-stage serous ovarian cancers. Experimental Design: Immunohistochemical localization of CD8 T lymphocytes was done on a homogeneous population of 184 high-grade, advanced-stage serous ovarian cancer tissue specimens. Microarray analysis was done on microdissected tumor epithelium from 38 specimens to identify genes up-regulated or down-regulated in specimens with differing numbers of tumor-infiltrating CD8 T lymphocytes. Quantitative real-time PCR and immunohistochemistry were used to validate a candidate gene. Univariate and multivariate survival analyses were done combining CD8T lymphocyte number and HLA-DMB expression with standard prognostic factors. Results: Marked CD8 T lymphocyte infiltration of the tumor epithelium is associated with a 20-month improvement in median overall survival. Additionally, when combined with cytoreduction status and age, CD8 T lymphocyte status is an independent prognostic factor for survival. Microarray analysis showed HLA-DMB, a component of the MHCII antigen presentation machinery, to be differentially expressed in specimens with an abundance of tumor-infiltrating CD8 T lymphocytes. This relationship was validated at both mRNA and protein levels. As well, high HLA-DMB expression in the tumor epithelium was associated with a significant improvement in median overall survival in both univariate and multivariate analyses. Conclusions: Tumor cell expression of HLA-DMB is associated with increased numbers of tumor-infiltrating CD8 T lymphocytes and both are associated with improved survival in advanced-stage serous ovarian cancer. © 2008 American Association for Cancer Research.