The goal of this multicentric French randomized trial is to determine the best auto fluorescence bronchial endoscopic follow-up strategy in high risk patients bearing low grade bronchial precancerous lesions. This Trial compares two follow-up modalities : observation without endoscopy for a three year period (group A " light " follow-up), versus repeated autofluorescence endoscopies and biopsies every six months for three years (group B " intensive ").