There is a growing experience with positron emission tomography (PET) in patients with pulmonary nodules or masses. As PET imaging becomes more widely available, it is important to thoughtfully define when application of this technology is warranted. Review of the literature to date suggests that PET imaging for diagnosis of pulmonary lesions is most useful in patients who have a low or intermediate risk of lung cancer as determined by an evaluation of symptoms, risk factors, and radiographic appearance. There is little role for PET in diagnosis in patients with a very low or a high risk of lung cancer, and there is little role in patients with lesions < 1 cm in diameter, or lesions suspected to be an infection, a bronchioloalveolar carcinoma, or a typical carcinoid tumor.