Ulcerative colitis is a chronic inflammatory disorder causing mucosal inflammation of the colorectum with crypt abnormality on biopsy. It affects the rectum and a variable extent of the colon in continuity. Ulcerative colitis is characterized by a relapsing and remitting course. It arises from an interaction between genetic and environmental factors, but the precise etiology is unknown. The incidence and prevalence in Korea are still low compared with those of Western countries, but have increased in recent years. There are many challenging issues on the diagnosis of ulcerative colitis, and sometimes these lead to differences in practice between clinicians. Therefore, IBD Study Group of KASID set out the Korean diagnostic guideline of ulcerative colitis. The diagnosis is based on clinical, endoscopic, radiologic, and histologic criteria. The symptoms are dependent upon the extent and severity of disease and most commonly include bloody diarrhea, rectal bleeding, and/or urgency. The systemic symptoms of malaise, tachycardia, fever, or weight loss are features of a severe attack. The laboratory findings may reveal leucocytosis, thrombocytosis, iron deficiency anemia, hypoalbuminemia, and elevated erythrocyte sedimentation rate and C-reactive protein indicating severe disease activity or chronicity. For the elimination of infectious causes, microbial investigation with stool specimens should be performed for common enteric pathogens including assays for Clostridium difficile toxin, and sometimes for amoeba or other parasites. The most typical endoscopic features are continuous, confluent, and concentric colonic involvement proximal to the anal verge. Endoscopic severity may be best well reflected by the presence of mucosal friability, spontaneous bleeding, and deep ulcerations. Typical pathologic findings are composed of widespread crypt architectural distortion (cryptitis, crypt abscess, and crypt atrophy), heavy, diffuse lamina propria cell infiltration, and basal plasmacytosis.