Background: To date, only limited data are available on genomic alterations present in metastatic lesions of patients with metastatic urothelial carcinoma (mUC). The constant release of cell free DNA (cfDNA) from tumor cells into the peripheral blood stream may provide a valuable source for detection of cancer associated somatic copy number variations and mutations. The aim of this project was to evaluate the feasibility of detecting clinically relevant genomic alterations in mUC by next-generation sequencing (NGS) of circulating cfDNA. Methods: Peripheral blood (20ml) was collected from 15 patients with mUC before chemotherapy. Plasma cfDNA was isolated using the QIAAmp Circulating Nucleic Acid Kit. Germline control DNA was extracted from peripheral blood mononuclear cells (PBMCs) using the DNEasy Kit (Qiagen). We performed deep (>500X) targeted DNA sequencing on cfDNA and germline DNA using a custom NimbleGen SeqCap EZ Choice Library and Illumina sequencers. Our custom panel included 50 bladder cancer-relevant genes, including key tumor suppressors (e.g. TP53, RB1, CDKN2A), cell cycle drivers (e.g. CCND1, CCNE1), DNA repair genes (e.g. ATM, BAP1, ERCC2), PI3K pathway genes (e.g. PIK3CA, PTEN, AKT1), and other oncogenic pathways (e.g. RAS/RAF, EGFR, ERBB2, PPARG, FGFR3). Results: Somatic mutations and/or copy number changes were detected in 14/15 samples, with a median variant allele frequency >5%. The overall mutational landscape was highly consistent with bladder cancer, including mutations in TP53 and ARID1A (and other chromatin modifiers), as well as hotspot activating mutations in PIK3CA. Typical copy number changes, including focal amplifications of ERBB2, KRAS, and CCNE1 were detected. We identified complex gene rearrangements, including an activating FGFR3 fusion in one case. Extended analyses with a larger patient population are currently pending. Conclusions: Non-invasive characterization of genomic changes in peripheral blood of patients with mUC using NGS of plasma cfDNA is feasible in a high proportion of patients. Analysis of cfDNA might therefore provide an improved understanding of genomic changes in mUC and could be developed as a biomarker to help guide the use of targeted therapies.