Background: MDSC have been linked to the chronic inflammatory microenvironment of tumor cells and pathologic outcomes in UC patients (pts) undergoing cystectomy. NLR is an established inflammatory biomarker with prognostic properties in mUC. We hypothesized that MDSCs correlate with NLR and OS in mUC. Methods: MDSCs were measured in blood samples from mUC patients by fresh unfractionated whole blood (WB) and peripheral blood mononuclear cells (PBMC). MDSCs were identified by flow cytometry in WB and defined as LinloCD33+/HLADR- (Total MDSC). MDSC subsets were defined as polymorphonuclear (PMN-MDSC: CD15+/CD14-), monocytic (M-MDSC: CD15-/CD14+), and uncommitted (UC-MDSC: CD15-/CD14-). MDSC populations were presented as % of live nucleated blood cells from PB and absolute numbers from WB. Spearman’s correlation assessed correlations between MDSC & NLR. Kaplan Meier curves and log rank test estimated OS from the time of MDSC collection to last follow up or date of death. Results: Of 79 pts, 77% were men and 42% were never smokers with a median age of 69 (31-83). Overall, 71% had pure UC and 81% had lower tract UC. Prior therapies include intravesical therapy (22%), neoadjuvant chemotherapy (31%), and cystectomy/nephroureterectomy (61%). Median follow up was 12 months (range: 0.6-36.5). PMN-MDSC was the predominant subset in WB and PBMC. There was significant correlation between individual MDSC subsets in WB and PBMC (p≤0.001). Negative correlation was noted between NLR and WB UC-MDSC:PMN-MDSC ratios (rho = -0.27, p = 0.03), as well as NLR and PB UC-MDSC:PMN-MDSC (rho = -0.28, p = 0.02). Median survival was 17.7 months (95% CI: 11.0-NA months). Overall 1-yr and 3-yr survival were 0.60 (95% CI: 0.49-0.73) and 0.15 (95% CI: 0.03-0.67), respectively. Higher WB UC-MDSC levels were associated with shorter OS (HR 2.85, 95% CI: 1.43-5.65, p = 0.003). Conclusions: Specific MDSC subsets correlate with NLR. Higher WB UC-MDSC levels have negative prognostic roles for OS. Given the feasibility of serial blood draws, dynamic assessment of MDSC over time and further validation with longer follow up are needed.