The poor outcome in symptomatic lung cancer patients and the much better prognosis when lung cancer is diagnosed and treated at early asymptomatic stages call for screening. As lung cancer predominantly affects smokers and individuals exposed to other carcinogens, screening programs need not include the whole population but only these risk groups. Every screening program will tend to better identify the more indolent tumours that grow slowly enough to be detected by screening before symptoms develop, whereas aggressive fast-growing tumours may present as interval cancers despite screening (length-time bias). Some malignant tumours detected with screening may never cause the person's death due to competing causes for death, particularly in heavy smokers, such as cardiovascular disease or other cancers (overdiagnosis bias). If a cancer is still lethal despite detection through screening, the affected individual may live longer with the diagnosis of cancer but not longer altogether (lead-time bias). It is likely that this will have a negative effect on that individual's quality of life. Participation in screening programs may have beneficial as well as adverse effects on smoking habits; in the worst case it may encourage people to continue smoking. Trials assessing chest radiography or sputum microscopy have not demonstrated a reduction in lung cancer mortality through screening, probably because the tests were not sensitive enough. computed tomography promises better sensitivity. Other modern tests such as fibre optic bronchoscopy, analysis of molecular markers or genetic testing in serum, sputum or exhaled air are not yet ready for clinical practice.