One hundred forty-two patients with active colonic or ileocolonic Crohn's disease were included in a multicenter prospective study. Data collection included 28 clinical, biological, and endoscopic items; the latter were recorded according to a standardized colonoscopic protocol; a previously validated endoscopic index of severity was calculated. Oral prednisolone (1 mg/kg body wt per day) was started and maintained until clinical remission and for at least 3 and at most 7 wk. A second clinical biological and endoscopic evaluation was then performed. At initial colonoscopy, mucosal lesions were, by decreasing order of frequency, superficial ulcerations, deep ulcerations, mucosal edema, erythema, pseudopolyps, aphthoid ulcers, ulcerated stenosis, and nonulcerated stenosis (93%, 74%, 48%, 44%, 41%, 35%, 10%, 8%, and 2% of cases, respectively). No correlation was found between the clinical activity index and any of the endoscopical data (lesion frequency and surface, endoscopic severity index). Ninety-two percent of patients underwent clinical remission within 7 wk of treatment. None of the 28 clinical biological and endoscopical items collected just before treatment could predict clinical response to steroids. Only 38 of the 131 patients in clinical remission were also in endoscopic remission. In conclusion, (a) the description and severity of colonoscopic lesions in active Crohn's disease have been quantified; (b) no correlation exists between clinical severity and nature, surface, or severity of endoscopic lesions; (c) Oral prednisolone (1 mg/kg body wt per day) induces a clinical remission in 92% of patients within 7 wk; (d) resistance to steroids cannot be predicted from the data collected before treatment onset; and (e) only 29% of patients in clinical remission also achieve endoscopic remission.