This randomised study aimed at evaluating the medical and economical impact of positron emission tomography (PET) using fluorine-18-fluoro-deoxyglucose (FDG) in the therapeutic prescription in patients with Hodgkin’s lymphoma or with large B and P cell non-Hodgkin’s lymphoma. The median progression free survival, the costs and the quality of life are compared between two groups : Group 1: the results of conventional staging and of the PET are known before stem cell transplantation. Group 2: only the results of conventional staging before stem cell transplantation are known before stem cell transplantation.