Autofluorescence endoscopy is a promising modality for diagnosis of colonic tumors. This article discusses the
origin of autofluorescence of the normal colon. Excised normal colons were analyzed by using fluorescence
stereomicroscopy and a fluorescence-lifetime microscopy system. Fluorescence images showed that the mucosa had
stronger autofluorescence than the submucosa. The results of fluorescence-lifetime measurement showed that
nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) might be responsible for the
autofluorescence of the colonic epithelia. Our results suggest that the mucosal autofluorescence generates by NADH and
FAD was an important source of the green autofluorescence.