Background: Primary carcinoma of the lung is the leading cause of cancer death in the United States. A test that detects early-stage disease could potentially reduce the number of deaths. A breath test for volatile organic compounds (VOCs) was sensitive and specific for primary lung cancer in two multicenter clinical studies. A third multicenter study confirmed these findings in cancer-free controls with a negative spiral CT of chest and patients with tissue-confirmed primary lung cancer. In a study of 451 subjects, a two-minute breath test for VOC biomarkers of lung cancer combined with fuzzy logic analysis of data predicted all TNM stages of disease with 84.6% sensitivity, 80.0% specificity and 0.88 area under curve (AUC) of the receiver operating characteristic (ROC) curve. Predictive accuracy was not affected by TNM stage of primary lung cancer, age, sex, or tobacco smoking. Biological basis of the breath test: Breath markers of primary lung cancer appear to emanate from other tissues, not from the tumor itself. Volatile products of oxidative stress are generated in all tissues of the body and excreted in the breath as alkanes and methylated alkanes. In patients with lung cancer, these VOCs may undergo accelerated catabolism, so that their altered abundance in breath provides a biomarker of disease. Alkanes and their metabolites are metabolized rapidly by high risk cytochrome P450 phenotypes (e.g., CYP1A1, CYP1A2 and CYP2E1) whose activity is induced by polycyclic aromatic hydrocarbons in tobacco smoke. This may explain why the breath test was abnormal in early (TNM stage I) disease, because induction of high risk cytochrome P450 phenotypes precedes the onset of neoplasia. Reason for the research: The clinical findings require confirmation in an additional clinical study in order for the Food &Drug Administration (FDA) to award the breath test pre-marketing approval. Experimental plans: We will perform a cross-sectional multicenter study to validate the methodology and the predictive algorithm that were previously developed. Breath tests will be performed in 400 anonymized subjects in order to compare cancer-free controls (with a negative spiral CT of chest) to patients with tissue-confirmed primary lung cancer. This will be a blinded study: breath samples will be analyzed at two independent laboratories blinded to the diagnosis, and a Data and Safety Monitoring Board will collate the predictions of the breath tests with the clinical diagnoses in order to determine the accuracy of the test. Long term aims: a. Scientific: To validate the previous clinical findings. b. Commercial: To obtain FDA pre-marketing approval (PMA) of the test in order to market it. Importance of the research: In clinical practice, the breath test could provide a safe and non-invasive ancillary test for primary detection of lung cancer. Its high negative predictive value (NPV) (>99%) could dependably exclude lung cancer in cancer-free normals. The breath test for biomarkers of lung cancer could provide a valuable new test that is inherently safe, non- invasive, and rational. Breath testing could potentially reduce the costs of early detection of lung cancer, and reduce the number of patients exposed to radiation from spiral CT of chest. Early, safe and cost-effective detection of lung cancer could potentially reduce the number of lives lost to the disease. We will perform a validation study of the breath test for biomarkers of lung cancer in order to obtain pre-marketing approval from the Food &Drug Administration. This breath test is inherently safe, non-invasive, and rational;it could reduce the costs of early detection of lung cancer, and reduce the number of patients exposed to radiation from spiral CT of chest. Early, safe and cost-effective detection of lung cancer could potentially reduce the number of lives lost to the disease.