One of the outstanding features of ulcerative colitis is the diversity of complications of the disease. They fall logically into two main groups: local complications in and around the large bowel, and remote or systemic complications affecting distant parts of the body. Some of these complications are so dangerous that they make a substantial contribution to the considerable fatality due to this disease. Tables XXIV and XXV show the incidence of the main complications divided between 'first attack' cases and 'relapses', and further subdivided according to whether the complication occurred during the first referred attack or during the period of subsequent follow-up.' The right-hand column shows the overall incidence of these complications in our total series of 624 patients. This incidence must be regarded as an underestimate for two distinct reasons. First, in many cases we have had to rely purely on the case notes for our information, and some of the complications, especially minor examples, may not have been recorded in all instances. Secondly, it can be seen from Tables XXIV and XXV that these complications are numerous during the course of follow-up and therefore some patients who have so far escaped complications are almost certain to have one or more in the future.