The study was conducted to ascertain whether the quantification of circulating cell-free DNA (cfDNA) in serum has value as a diagnostic or for monitoring the progression of non-small cell lung cancer (NSCLC). The serum/plasma cfDNA concentration was quantified by absolute qPCR of long interspersed nuclear element-1 (LINE1) in 60 NSCLC patients and 68 controls in good health. Receiver operating characteristic (ROC) curve analysis was performed to determine the diagnostic utility and cut-off levels of cfDNA, CEA, and CYFRA21-1 in NSCLC patients. Correlations between cfDNA and age, sex, tumour stage and progression-free survival (PFS) were analysed. A follow-up study was conducted on 4 NSCLC patients, and serum cfDNA, CEA, and CYFRA21-1 were quantified throughout disease progression. Serum cfDNA levels were significantly higher in NSCLC patients than those in normal controls. Elevated serum cfDNA concentration was also significantly associated with advanced tumour stage. Serum cfDNA had a ROC area under the curve comparable to that of CEA and CYFRA21-1 for the diagnosis of NSCLC, and the combined cfDNA/CEA/CYFRA21-1 indicator had the highest diagnostic efficiency. Moreover, increased serum cfDNA levels were strongly correlated with tumour progression and poor PFS. This study preliminarily confirmed that cfDNA can monitor disease progression in NSCLC patients, and the lead time was 1-7 months compared with clinical medical imaging. Serum cfDNA may be useful in monitoring NSCLC progression, suggesting that the non-invasive quantification of serum cfDNA by LINE1 qPCR is a viable option for predicting progression and disease severity when repeated invasive tissue biopsy is not possible.