Colonoscopy was performed on acute stage within 72 hours from onset in 48 patients with antibiotic-associated hemorrhagic colitis (AAHC). From our observations we have developed a new endoscopic classification of AAHC and investigated the incidence of each finding in further detail. We have classified in principle the endoscopic findings of the 48 subjects into two groups: major and minor ones respectively. Category of major findings were: 1) diffuse mucosal hemorrhage (100%); 2) spotty mucosal hemorrhage (100%), and 3) linear mucosal hemorrhage (22.9%) w, hile minor findings were: 1) irregular ulcers in 10.4%; 2) aphthoid ulcers in 6.3%, a nd 3) linear erosions or ulcers in 4%. Minor findings were ulcers or erosions present over the hemorrhagic mucosa associated with the moderate degree of inflammation. A histopathologic study of colon biopsy specimens from 24 patients with AAHC showed hemorrhage and inflammatory cell infiltration in the lamina propria mucosae varing from mild to moderate in extent. It was concluded that AAHC was a colonic mucosal hemorrhagic disease caused by the destruction of mucosal vessels from unknown causes and in this disease mild to moderate inflammation was partially followed by ulceration over the edematous and hemorrhagic mucosa of the colon.