The outcome of collaboration between INSERM (National Institute for Health and Medical Research), SFR (French Radiological Society) and SPLF (French Language Society of Pneumology), DEPISCAN is a pilot study for a large randomised trial which aims to evaluate screening for broncho-pulmonary cancer by the low dose spiral CT scanner. The principal objective of this preliminary study is to test the feasibility of a randomised study comparing the CT scanner and the chest x-ray in 1,000 smokers and ex-smokers aged over 50 and followed up for 2 years. Recruitment of subjects will be undertaken by 130 general practitioners based on 14 hospital centres distributed throughout France. We will evaluate the safety of the patients and the effect of screening on their quality of life and their tobacco consumption as well as the economic and methodological aspects of the study.