This observational case-control study will involve a random selection from a group of smokers who are participating in the Prostrate, Lung, Colon, and Ovarian Cancer (PLCO) Screen Trial. The chosen cases will include 300 incident lung cancer cases and 300 controls (participants who have had no diagnosis of lung cancer). Demographic and baseline data from the PLCO database will be obtained. Prior baseline blood samples from the PLCO trial will be obtained as well. Based on age, sex, and smoking history, participants will be grouped into triplets in order to pool their blood samples. These samples will then be analyzed to determine whether distributions of biomarker levels in lung cancer participants differ from those in non-lung cancer participants. This study will not involve recruitment of any participants, as data and samples from the PLCO trial will be used and no new blood samples will be obtained.