The study of the extension of primitive lung cancer involves, on one hand, the determination of the locoregional extension concerning the tumator and adenopathies, and on the other hand, the determination of the metastatic extension. A careful clinical examination determines the need for complementary examinations providing decisive information for or against interventions. Systematic lung endoscopy should consist of staged biopsies. Mediastinoscopy estimates the locoregional extension in a more satisfactory manner than angiography or lung scintigraphies. The search for bone or cerebral metastasis is often negative in the absence of clinical symptomology in spite of the recent contribution of tomodensitometry. The detection of abdominal metastasis by biological examination, scintigraphies or contrast X-rays is liable to interpretational errors in one-third of the cases. It is for this reason that we preconize laparotomy before lung exeresis. In our series, among 175 laparotomies, 35 revealed abdominal metastasis whose discovery enables us to avoid useless, if not harmful, thoracic surgery.