The diagnostic work-up of ulcerative colitis at presentation is based on the collection of clinical, microbiological, radiological, endoscopic and histologic data. Serological markers are characterized by too low a sensitivity to be commonly utilized in clinical practice. Although endoscopic and histologic features are characterized by very high sensitivity and specificity for the diagnosis of ulcerative colitis, negative stool cultures and parasites are mandatory to exclude an infectious aetiology at presentation. The treatment of choice of an acute flare-up of distal ulcerative colitis is represented by oral or topical mesalazine, or a combination of both, whereas the use of topical or systemic steroids should be restricted to patients who prove to be refractory to first-line treatments. Preliminary data suggest that the achievement of endoscopic and histologic remission after an acute flare of the disease might be associated with a prolonged remission.