15-40% of non-small cell lung cancer (NSCLC) patients harbor epidermal growth factor receptor (EGFR)-sensitizing mutations. Tyrosine kinase inhibitors (TKIs) provide significant clinical benefit in this population, yet all patients will ultimately progress. Liquid biopsy can reliably identify somatic tumor-associated EGFR mutations in plasma. This study aimed to assess the feasibility and value of the quantitative assessment of EGFR driver mutations in plasma in EGFR-mutated NSCLC patients treated with EGFR-TKIs as a tool to evaluate therapeutic response to TKIs and monitor for disease progression. The study included 136 patients with tissue biopsy-confirmed EGFR-sensitizing, mutation-positive lung adenocarcinoma with plasma collected prior to TKI treatment and at least two post-initiation TKI treatment/follow-up blood samples. Plasma samples were tested with the cobas® EGFR Mutation Test v2 (cobas EGFR Test), and semi-quantitative index (SQI) values for each identified mutation were reported by the assay software. The most common baseline EGFR mutations detected in tissue were L858R (53.7%) and exon 19 deletion (39.7%). Plasma cell-free DNA analysis detected EGFR mutations in 74% of the baseline samples. Objective response rate by RECIST 1.1 was achieved in 72% of patients, while 93% had a molecular response (defined as disappearance of the EGFR mutation from plasma). 83% of patients had molecular progression (MP; 1.5X SQI increase or new T790M mutation), and 82% who had a clinical response had clinical progression. On average, MP occurred 42 days prior to clinical progression. Patients who progressed while on first-line TKI showed MP of the original EGFR-sensitizing mutations prior to the emergence of a T790M mutation, which was detected in 27% of the EGFR plasma-positive patients. Longitudinal monitoring of EGFR mutational load in plasma is feasible and can predict both response and clinical progression in EGFR-mutated NSCLC patients treated with EGFR-TKIs, as well as detect treatment-emergent EGFR mutations.