Regulatory T cells (Tregs) and M2 macrophages have been hypothesized to contribute to tumor progression. We found that M2 macrophages and Tregs are associated with more aggressive renal cell carcinoma, and that they have a synergistic effect on clinical outcome. Background: It has been hypothesized that M2 macrophages and regulatory T cells (Tregs) may contribute to tumor progression by suppression of antitumor immunity. Objective: To investigate the association between infiltration of CD163+ M2 macrophages and CD4+FOXP3+ Tregs with clinical outcomes in renal cell carcinoma patients. Design, setting, and participants: A cohort of 346 patients diagnosed with renal cell carcinoma at Örebro University Hospital between 1986 and 2011 was evaluated for CD163+ M2 macrophage and CD4+FOXP3+ Treg infiltration by immunohistochemistry. Outcome measurements and statistical analysis: Associations between clinicopathological features and infiltration of CD163+ M2 macrophages and/or CD4+FOXP3+ Tregs were estimated with chi-square or Fisher's exact tests. For survival analyses, Kaplan-Meier curves with log-rank tests and multivariate Cox proportional hazards regression models were used. Results and limitations: We found that infiltration of CD163+ M2 macrophages and CD4+FOXP3+ Tregs were associated with adverse clinical outcomes. Our data further demonstrate that CD163+ M2 macrophages and CD4+FOXP3+ Tregs colocalize in tumor and normal tissue, and that this colocalization may have synergistic effects on tumor aggressiveness. The use of tissue microarrays rather than whole sections may be viewed as a limitation. Conclusions: Infiltration of CD163+ M2 macrophages and CD4+FOXP3+ Tregs is associated with recurrence of renal cell carcinoma, and colocalization of these cell types may have an association with clinical outcome. Patient summary: The aim of this study was to investigate the association between infiltration of M2 macrophages and regulatory T cells with clinical outcomes in renal cell carcinoma. We demonstrated that renal cell carcinoma patients with high infiltration of both these cell types are at an increased risk of poor clinical outcomes.