Background: Fibroblast growth factor receptor (FGFR) inhibitors (e.g., erdafitinib) are increasingly important in the management of FGFR-mutated urothelial carcinoma. FDA-approved archival tissue testing for specific FGFR alterations was implemented as a companion diagnostic for erdafitinib. However, longitudinal sequencing studies indicate variable tumor FGFR status over time, and erdafitinib resistance mechanisms in metastatic urothelial carcinoma (mUC) are underreported. This ongoing study aims to evaluate the accuracy of cell-free DNA (cfDNA) compared to archival tissue testing in mUC for FGFR alterations detection, and to evaluate genomic mechanisms of erdafitinib resistance in cfDNA at progression. Methods: Patients with progressing mUC who were undergoing archival tissue testing for FGFR1-3 mutations and/or fusions and who had blood samples drawn during the management of their metastatic disease were eligible. Plasma cfDNA and matched leukocyte DNA were subjected to deep targeted sequencing with a custom panel including UC-specific gene loci and all clinically approved hotspots in FGFR1+2 and all exons and introns of FGFR3. Results: As of January 2023, 109 patients from 6 sites were enrolled. Median age at diagnosis was 71, 33% had upper urinary tract primaries, and 76% were male. Tissue and cfDNA results for comparison were available for 69 patients to date. Actionable somatic FGFR alterations were found in the tissue of 15 patients (31%); the most common alteration was the FGFR3 p.S249C mutation (67%). 50 of the analyzed cfDNA samples had detectable somatic circulating tumor DNA (ctDNA) variant allele fraction of 0.5% (72%). Of those, 49 had an evaluable tissue test result. Analysis of this subset revealed high concordance (92%) between the two test methods. With the assumption that archival tissue testing is considered the ‘gold standard’, sensitivity is 93% and specificity is 91%. The four discordant results comprised one cfDNA test with undetectable FGFR3-TACC3 fusion, which was detected in tissue and three positive ctDNA test results in patients with FGFR wild-type tissue tests. In one case at erdafitinib progression, ctDNA revealed multiple subclonal populations with distinct FGFR3 gatekeeper mutations suggesting polyclonal resistance. Conclusions: This ongoing study suggests cfDNA is a valuable minimally invasive adjunct to tissue-based assays for the detection of FGFR alterations to identify patients for FGFR inhibitor therapy and to monitor for mechanisms of resistance.