Background: ctDNA monitoring has shown promising results in predicting relapse in resected solid tumors. Its role in treatment response assessment and predicting survival outcomes in the unresectable or metastatic disease setting merits further investigation. In our study, we attempt to assess the role of early ctDNA changes in predicting disease response, progression, and overall survival outcomes in pts with advanced stage melanoma/skin cancer treated with ICI therapy. Methods: A retrospective analysis using a personalized, tumor-informed ctDNA assay (Natera) on prospectively collected plasma samples from pts with unresectable stage III/IV melanoma/skin cancer treated with anti-PD-1 based therapy at the University of Wisconsin (Madison) was performed. Baseline ctDNA levels were assessed prior to the start of treatment and at 3-4 weeks (i.e. prior to the second treatment dose). A logistic regression model was used to evaluate the odds of overall disease control [complete response + partial response + stable disease, per RECIST version 1.1] based on the change in ctDNA levels (decrease vs increase) between both time points. Cox proportional hazard models were used to investigate the effects of ctDNA level change on progression free survival (PFS) and overall survival (OS). Results: 46 pts were evaluated. 82% melanoma, 14% Merkel cell carcinoma, 2% skin adenocarcinoma and 2% squamous cell carcinoma. 77% were treated with dual ICI (anti-PD-1 based) therapy and 23% with anti-PD-1 monotherapy. Median follow up was 12.1 months. Median change in ctDNA levels from baseline were -4.83 MTM/mL among pts with ctDNA decrease and +37.39 MTM/mL among pts with ctDNA increase. A qualitative decrease in ctDNA level was associated with overall disease control (OR 65.00, 95% CI 11.88-587.89, p<0.0001), longer PFS (HR 0.08, 95% CI 0.03-0.22, p<0.0001), and longer OS (HR 0.17, 95% CI 0.05-0.54, p=0.0008) compared to an increase in ctDNA level from baseline to 3-4 weeks after starting ICI therapy. Median PFS was not reached (NR) and 1.63 months; and median OS was NR and 5.57 months among pts with ctDNA decrease and ctDNA increase, respectively. Conclusions: We found that early ctDNA dynamics after 3-4 weeks of ICI initiation in pts with advanced melanoma/skin cancer appears to be a candidate strategy to predict treatment response, risk of progression, and potentially long-term survival. Larger prospective studies are warranted to validate the utility of ctDNA in treatment monitoring.