Diarrhea "with" bacterial fermentation is characterized by acidic liquid stools containing high amounts of organic acids. Disaccharide malabsorption is the main cause. The mechanism of diarrhea is osmotic, and colonic fermentations reduce diarrhea. It is unlikely that starch malabsorption induces significant diarrhea, whereas a high-fiber diet is responsible for "physiologic" diarrhea. Colonic fermentations increase diarrhea due to organic colitis and the "motor diarrheas". They may be responsible for some intestinal symptoms in patients with irritable bowel syndrome. This does not imply a "hyperfermentative" process due to a hypothetical disturbance of colonic microbial ecology.