In a prospective study of 34 patients with active ulcerative colitis, the findings of inflammation on plain abdominal films and air enema films were compared to those at colonoscopy including biopsy within 10 days. The degree of inflammation on X-ray films was graded independently by two radiologists, at colonoscopy by one gastroenterologist and from histological slides from 6 different colon segments by one pathologist for each patient. Air enema films had a high sensitivity for endoscopically confirmed friable or ulcerated mucosa (0.91). There was a high specificity (0.86) when excluding inflammation in individual colon segments. Absence of fecal residue as an indication of active inflammation had the same positive predictive value, 0.95, as an abnormal air enema film, 0.98 for endoscopically confirmed inflamed mucosa. The presence of fecal residue or a normal air enema film excluded a friable or ulcerated mucosa at endoscopy with negative predictive values of 0.83 and 0.86, respectively. Patients who had had a complete colonoscopy (n = 16) were divided into groups with total, extensive or distal colitis. Air enema films underestimated the extent of inflammation in 8 of 16 patients compared to colonoscopy. Of 6 patients with distal disease only on air enema films, 5 had disease above the splenic flexure at endoscopy. In patients with ulcerative colitis (1) the presence of fecal residue and a normal air enema film exclude a friable or ulcerated mucosa with a high degree of certainty, and (2) the absence of fecal residue and an abnormal air enema film are predictors of the presence of endoscopically confirmed inflammation.