The diagnostic accuracy of abdominal ultrasonography in inflammatory bowel disease (IBD) has been evaluated in a prospective, randomized, blind study. A total of 181 patients (89 with Crohn's disease, 57 ulcerative colitis and 35 controls) were examined. Sensitivity and specificity of diagnosis in Crohn's disease, corrected for prevalence, were 80.8 and 79.2%, respectively, and a very similar accuracy was found in the differential diagnosis of IBD. In conclusion, ultrasonography can play a role in detecting Crohn's disease and in the differential diagnosis of chronic IBD.