Lung cancer is the leading cause of cancer death in the United States, but no scientific organization currently recommends screening because of limited evidence for its effectiveness. Despite this, physicians often order screening tests such as chest X-rays and computerized tomography scans for their patients. Limited information is available about how physicians decide when to order these tests. To identify factors that affect whether physicians' screen patients for lung cancer, we conducted five 75-min telephone-based focus groups with 28 US primary care physicians and used inductive qualitative research methods to analyze their responses. We identified seven factors that influenced these physicians' decisions about screening patients for lung cancer: (1) their perception of a screening test's effectiveness, (2) their attitude toward recommended screening guidelines, (3) their practice experience, (4) their perception of a patient's risk for lung cancer, (5) reimbursement and payment for screening, (6) their concern about litigation, and (7) whether a patient requested screening. Because these factors may have conflicting effects on physicians' decisions to order screening tests, physicians may struggle in determining when screening for lung cancer is appropriate. We recommend (1) more clinician education, beginning in medical school, about the existing evidence related to lung cancer screening, with emphasis on the benefit of and training in tobacco use prevention and cessation, (2) more patient education about the benefits and limitations of screening, (3) further studies about the effect of patients' requests to be screened on physicians' decisions to order screening tests, and (4) larger, quantitative studies to follow up on our formative data.