RATIONALE: Free circulating plasma DNA has emerged as a potential biomarker for early lung cancer detection. In a previous case-control study we have shown that high levels of plasma DNA are a strong risk factor for lung cancer. OBJECTIVES: To assess the diagnostic performance and prognostic value of plasma DNA levels in a cohort of 1,035 heavy smokers monitored by annual spiral computed tomography (CT) for 5 years. METHODS: Plasma DNA levels were determined through real-time quantitative PCR at baseline and at time of lung cancer diagnosis. Screening performance of the assay was calculated through the area under the receiver-operating characteristic curve (AUC-ROC). Kaplan-Meier analyses were computed for association with prognosis. MEASUREMENTS AND MAIN RESULTS: Median baseline concentration of plasma DNA was not different in individuals who developed CT-detected lung cancers in the 5-year period (n = 38) versus cancer-free control subjects (AUC-ROC, 0.496; P = 0.9330), and only slightly higher at the time of cancer diagnosis (AUC-ROC, 0.607; P = 0.0369). At surgery, plasma DNA was higher in tumors detected at baseline (AUC-ROC, 0.80; P < 0.0001) and in Stage II to IV tumors detected during the first 2 years of screening (AUC-ROC, 0.87; P < 0.0001). A longitudinal study of plasma DNA levels showed increased values approaching to lung cancer diagnosis (P = 0.0010). Higher plasma DNA was significantly associated with poorer 5-year survival (P = 0.0066). CONCLUSIONS: Baseline assessment of plasma DNA level does not improve the accuracy of lung cancer screening by spiral CT in heavy smokers. Higher levels of plasma DNA at surgery might represent a risk factor for aggressive disease.