Background: Next-generation sequencing (NGS) of cfDNA is an emerging non-invasive strategy to define tumor mutation profiles that counters spatial and temporal limitations of sequencing single tissue specimens. We examined the feasibility of NGS of cfDNA in mUC and compared mutation profiles from cfDNA to results of tissue NGS previously performed in the clinical setting. Methods: Plasma cfDNA was collected in mUC pts and analyzed using a capture-based NGS assay (MSK-IMPACT) targeting 341-468 genes. NGS profiles from cfDNA and archival tumor tissue (using the same assay) were analyzed in parallel with an established bioinformatics pipeline to identify somatic variants. Results: In 26 pts, NGS analysis of cfDNA detected ≥1 somatic mutations (range 1-21) in 69% (18/26). For 15 pts, NGS data was available from archival tissue (11 primary tumors, 3 metastases, and matched primary/metastatic tissue in 1 case). The interval between cfDNA and tissue collection ranged from 35 days to > 4 yrs. 73% (11/15) of pts received intervening treatment, including 47% (7/15) with chemotherapy, 67% (10/15) with immunotherapy, and 40% (6/15) with both. In 40% (6/15), cfDNA harbored alterations not found in archival tumor tissue. In 73% (11/15), some mutations within archival tissue were not detected in cfDNA, including hotspot HER2 S310F and FGFR3 S249C mutations. Tumor and cfDNA mutation profiles were identical in 20% (3/15), with the tumor/cfDNA interval in this group ranging from 35 days to < 1.5 yrs. Somatic alterations including hotspot ERCC2 P463A and PIK3CA E545K mutations were detected in cfDNA from 3 pts where archival tumor tissue NGS failed. Thus, cfDNA identified new mutations in 50% (9/18) of pts for whom cfDNA identified somatic mutations and tissue NGS was previously attempted. Conclusions: NGS of cfDNA from mUC pts is feasible and successfully detected actionable alterations when archival tumor sequencing failed. The differences between tumor and cfDNA mutation profiles in many pts may reflect tumor evolution or intratumor heterogeneity. Mutation profiles in mUC may be incompletely assessed with NGS of archival tissue, and further investigation of plasma cfDNA for genomic profiling is warranted.