Ninety-two lung cancers were detected in the Mayo Lung Project in patients undergoing chest radiography every four months for screening. Fifty patients had a peripheral nodule, 16 had a perihilar nodule, 20 had hilar or mediastinal enlargement, and six had pneumonitis. The peripheral cancers grew slowly. Ninety per cent were visible in retrospect for months or even years. Despite this, 70% of the peripheral cancers were classified as postsurgical American Joint Committee (AJC) Stage 1. The central cancers grew rapidly, usually presenting as hilar or mediastinal enlargements after normal findings on the previous radiograph obtained four months earlier. Most were classified as AJC Stage 3 tumors.