NLST participants were randomized to either low-dose helical CT or chest x-ray in equal proportions. A total of 53,454 participants were enrolled (26,722 in low-dose CT and 26,732 in chest radiography) at 33 screening centers across the United States. Screening was offered three times (at baseline and two annual follow-up examinations). The primary endpoint of the study was lung cancer mortality. The study arms were compared with regard to overall mortality, lung cancer incidence, and screening-related complications.