Background: Recent studies indicate that PD-1 and PD-L1 checkpoint inhibitors have activity in chemotherapy refractory patients with muscle invasive (MIBC) and metastatic (mBCa) bladder cancer. PD-L1 expression on tumor cells or lymphocytes may correlate with response to therapy. To identify potential patients who may benefit from PD-1/PD-L1 targeted immunotherapeutics, we utilized a non-invasive blood test to evaluate PD-L1 protein expression in CTCs and WBCs of bladder cancer patients. Methods: Whole blood from 20 patients with MIBC or mBCa were collected and shipped to Epic Sciences. All nucleated cells were plated onto glass slides and subjected to immunofluorescent (IF) staining and CTC identification using scanners and algorithmic analysis. CTCs, defined as traditional (CK+ CD45- w/ intact DAPI+ nuclei and morphologically distinct) or CK- (CK-, CD45-, DAPI+, intact and distinct) were identified. PD-L1 biomarker characterization was assessed by IF staining. UroVysion FISH testing was used to assess genomic abnormalities in a subset of patient samples. Additionally, WBCs (CD45+ cells) were assessed for PD-L1 expression, relative to healthy controls. Results: Eighty percent of patients had mBCa and 20% had MIBC, 70% percent were men, and themedian age was 67 years, (range 43 to 88). Traditional CTCs were detected in 11/20 (55%) patients. Seven out of 20 (35%) patients had PD-L1+ cells, 4 of these patients had exclusively CK-/PD-L1+ CTCs, a subset of which were confirmed as malignant via FISH. CK- CTCs were detected in 14/20 (70%) patients. Five patients had greater than 4-fold PD-L1 positivity in WBCs as compared to healthy donors. Conclusions: Patients with MIBC and mBCa patients have detectable, heterogeneous CTCs with a population of CK-/PD-L1+ CTCs. Utilization of a liquid biopsy to identify patients with PD-L1+ CTCs and PD-L1+ WBCs may enable patient selection or short term therapeutic monitoring for measuring therapeutic efficacy. Further work will investigate association of PD-L1+ CTCs and WBCs with clinical response to PD-1 checkpoint immunotherapy.