PURPOSE: Decisions to continue or suspend therapy with immune checkpoint inhibitors are commonly guided by tumor dynamics seen on serial imaging. However, immunotherapy responses are uniquely challenging to interpret because tumors often shrink slowly or can appear transiently enlarged due to inflammation. We hypothesized that monitoring tumor cell death in real-time by quantifying changes in circulating tumor DNA (ctDNA) levels could enable early assessment of immunotherapy efficacy. Experimental Design: We compared longitudinal changes in ctDNA levels with changes in radiographic tumor size and with survival outcomes in 28 metastatic non-small cell lung cancer patients receiving immune checkpoint inhibitor therapy. CtDNA was quantified by determining the allele fraction of cancer-associated somatic mutations in plasma using a multi-gene next-generation sequencing assay. We defined a ctDNA response as a >50% decrease in mutant allele fraction from baseline, with a second confirmatory measurement. RESULTS: Strong agreement was observed between ctDNA response and radiographic response (Cohen's kappa, 0.753). Median time to initial response among patients who achieved responses in both categories was 24.5 days by ctDNA vs. 72.5 days by imaging. Time on treatment was significantly longer for ctDNA responders vs. non-responders (median 205.5 vs. 69 days; P<0.001). A ctDNA response was associated with superior progression-free survival (hazard ratio [HR], 0.29; 95% CI, 0.09-0.89; P=0.03), and superior overall survival (HR, 0.17; 95% CI, 0.05-0.62; P=0.007). CONCLUSIONS: A drop in ctDNA level is an early marker of therapeutic efficacy and predicts prolonged survival in patients treated with immune checkpoint inhibitors for non-small cell lung cancer.