INTRODUCTION: Molecular residual disease detected by circulating tumor DNA (ctDNA) has been reported to be predictive of patients' outcomes in various types of cancers after curative intent treatment. Nevertheless, additional detailed information regarding the association of longitudinal ctDNA detection with long-term follow-up in lung cancer is needed. Here, we report on a cohort of patients with NSCLC who underwent definitive surgery and ctDNA analysis in the pre-operative, adjuvant, and surveillance settings. METHOD: Plasma samples were collected from 46 patients with clinical stage II-III NSCLC before surgery (n = 46), after surgery (n = 45), and every six months until two years thereafter (n = 78). A clinically validated, personalized, tumor-informed 16-plex polymerase chain reaction-next-generation sequencing assay was used for the detection and quantification of ctDNA in retrospectively analyzed plasma samples. RESULTS: Circulating tumor DNA was detected in the first postoperative (within 51 days after surgery) plasma samples in 13% (6/45) of patients (landmark analysis). All of them had disease recurrence within a median of 9.1 months. These patients had shorter recurrence-free and overall survivals than those without detectable ctDNA at a landmark time point ( CONCLUSIONS: The presence of ctDNA post-surgery or during surveillance identifies patients with NSCLC at high risk of recurrence. Serial testing is important to detect disease recurrence earlier (lead-time: 3.2 months).