Antibiotics related colitis is a well recognized disease entity which in its severest form may result in pseudomembranous colitis, whereas in another form, acute hemorrhagic colitis without pseudomembrane, related to the use of penicillin-type antibiotics is rarely reported. The clinical features of hemorrhagic colitis associated with antibiotics was characterized that the bloody diarrhea, often with abdominal cramping pain begins 2-7 days after starting the antibiotics and rapidly recovered after its withdrawal. Pathogenesis of this disease is not entirely clear. It has been believed that right-sided hemorrhagic colitis is one of the main forms of colitis associated with antibiotics, especially ampicillin derivatives or cephalosporin, but recent reports presented left-sided colitis. We experienced 2 cases of hemorrhagic colitis developed on the left colon after the introduction of quinolone. (Korean J Gastrointest Endosc 2001;22:45 - 49)