During a ten-year period, a double-blind retrospective study of 32 colectomy specimens from patients with inflammatory bowel disease (IBD) showed that the majority of cases could be clearly separated into ulcerative colitis (UC, 65%) and Crohn's disease (CD, 19%). However, in five (16%) colectomy specimens, the pathologic changes did not fulfill the criteria generally accepted for UC and CD. Criteria were laid down to differentiate the indeterminate form of colitis from the two more familiar types of IBD. We discuss the value of the category "indeterminate colitis" and emphasize that the term "transmural inflammation" is loosely used and that accurate definition of this criterion removes much of the difficulty from the differential diagnosis of IBD.