We evaluated whether the amount of circulating DNA in plasma could discriminate between lung cancer patients and healthy individuals and whether it is related to disease progression, and we analyzed the kinetics of plasma DNA in disease-free, surgically resected patients. Plasma DNA quantification and analysis of microsatellite alterations were performed in a consecutive series of 84 patients with non-small cell lung cancer, who were studied during follow-up, and 43 healthy controls. In patients, the mean values of plasma DNA concentration were higher than in controls even considering stage Ia patients. Sensitivity and specificity estimates were calculated as the area under the receiver operating characteristic curve (AUC-ROC) curve and showed a value of 0.844. Variations in DNA level and in microsatellite changes correlated with the clinical status of 38 patients monitored during follow-up. The data suggest that quantification and molecular characterization of plasma DNA in lung cancer patients are valuable noninvasive diagnostic tools for discriminating patients from unaffected individuals and for detecting early recurrence during follow-up.