Twenty children with active ulcerative colitis were assessed before and after 8 weeks of medical therapy with 5-aminosalicylic acid (5-ASA) derivatives and corticosteroids. Local therapy was given for distal disease (seven cases); other disease was treated with oral prednisolone (1-2 mg/kg/day, maximum 40 mg). Eighteen of the children showed a clinical improvement on therapy, and complete remission of clinical disease activity by 8 weeks was seen in 17 (85%). C-reactive protein was elevated initially in 10 of 20 children and returned to normal posttreatment in all but one. Reassessment of the colon after treatment showed an improved endoscopic appearance in 15 and complete remission in eight (40%). Histological improvement was seen in 13, with full remission in only three (15%). In conclusion, remission of clinical disease activity by corticosteroid therapy in ulcerative colitis may not be accompanied by endoscopic remission and uncommonly by mucosal healing. This finding may be important prognostically because of the risk of dysplasia in long-standing persistent mucosal inflammation.