Ulcerative colitis and granulomatous colitis are distinct entities, but up to 10 per cent of colectomy specimens remain unclassified. Ulcerative colitis is primarily a mucosal disease, and other changes appear to be secondary to this process. By contrast, Crohn's disease, or granulomatous colitis, involves the whole thickness of the bowel wall. About 20 per cent of the cases of Crohn's disease involve the small and large bowel, while another 20 per cent are restricted to the large bowel. Since granulomatous colitis is a patchy disease, and many of the changes are deep within the bowel wall, rectal biopsy may not be as helpful as in ulcerative colitis. Fully developed granulomas are present in only a small minority of cases, and a diagnostic report of granulomatous colitis may be given in the absence of granulomas. In biopsy material, the differentiation of inflammatory bowel disease from ischemic colitis and pseudomembranous colitis may be difficult. In the absence of specific demonstration of an organism it may also be impossible on rectal biopsy to distinguish amebic or bacillary dysentery from ulcerative colitis. Even by colectomy, 29 of 300 specimens were sufficiently atypical so as not to warrant a label of Crohn's disease, or ulcerative colitis. Cancer of the colon, which is common in ulcerative colitis, is rare in Crohn's disease, but may also represent a definite complication in the latter. Immunologic studies are still confusing, but it is suggested that patients with ulcerative colitis and Crohn's disease may have a state of altered immunologic reactivity.