Acute unclassified colitis could be the first attack of inflammatory bowel disease, particularly chronic ulcerative colitis or acute non specific colitis regarded as being of infectious origin without recurrence. The aim of this work was to determine the outcome of 104 incidental cases of acute unclassified colitis diagnosed during the year 1988 at a census point made 2.5 to 3 years later and to search for demographic and clinical discriminating data for final diagnosis. Thirteen patients (12.5%) were lost to follow up. Another final diagnosis was made in three other patients: two had salmonellosis and one diverticulosis. Of the remaining 88 patients, 46 (52.3%) relapsed and were subsequently classified as inflammatory bowel disease: 54% ulcerative colitis, 33% Crohn's disease and 13% chronic unclassified colitis. Forty-two (47.7%) did not relapse and were considered to have acute non specific colitis. The mean age at onset was significantly lower in patients with inflammatory bowel disease (32.3 years) than in patients with acute non specific colitis (42.6 years) (P < 0.001). No clinical data (diarrhea, abdominal pain, bloody stool, mucus discharge fever, weight loss) was predictive of the final diagnosis. In this series, 52.3% of patients initially classified as having an acute unclassified colitis had a final diagnosis of inflammatory bowel disease after a 2.5-3 years follow-up. These data warrant a thorough follow up of acute unclassified colitis, especially when it occurs in patients < 40 years.