The possibility that the small intestine may represent a reservoir for Clostridium difficile was studied, using segments of human jejunum collected at necropsy. Our results (three of 100 specimens positive for C difficile culture) support the hypothesis that C difficile can be found in human jejunum and that it adheres to the normal mucosa as a resident bacterium. These findings suggest that gastrointestinal disease caused by C difficile has an endogenous origin.