Records of 1084 patients entered into the National Cooperative Crohn's Disease Study were analyzed to gather information concerning the natural history and clinical features of Crohn's disease. The age of onset reached a single peak between the second and fourth decade and was evenly distributed in both sexes. There was an average interval of 35 mo from onset of symptoms to diagnosis. Involvement of both colon and terminal ileum was the most frequent pattern and was present in 55% of patients. The disease was confined to the terminal ileum, other areas of the small intestine, or colon-only in 14%, 3%, and 15% of patients, respectively. Sigmoidoscopic abnormalities were seen in 34% of all patients and 51% of patients with Crohn's colitis. Diarrhea, abdominal pain, weight loss, and fever were present in the majority of the patients. Lower GI bleeding, fever, and perianal complications characterized patients with colon-only involvement. The frequency of extra intestinal manifestations was similar in all groups. Among patients who were randomized to placebo, 32% achieved a spontaneous remission by the end of 17 wk, and 53% of these were still in remission at the end of 24 mo. Clinical remission was associated with an improvement in barium x-rays in 18% of the patients. The predicted factors associated with favorable outcome in placebo-treated patients were: previous surgical removal of all observable disease, absence of perianal disease, and Crohn's Disease Activity Index value under 200.