The colonic mucosa produces a protective and lubricating layer of mucus. In certain conditions, the quantity and quality of this mucus is impaired. This study assessed the histochemical changes in mucus in inflammatory bowel disease compared with the severity and extent of the condition. Biopsy specimens were taken from 62 patients (32 with ulcerative colitis; ten with colonic Crohn's disease; ten with diverticular disease; ten with normal controls) and sections stained with high iron diamine-alcian blue to distinguish sulphated mucins from sialomucins. Normal subjects showed a predominance of sulphated mucins. The patients with Crohn's and diverticular disease also demonstrated this normal pattern. Of the 20 patients with ulcerative colitis, and without demonstrable dysplastic changes, only one showed a moderate increase in sialomucins. However, of the 12 patients with extensive colitis and dysplastic changes, ten had an increase in sialomucins. Thus, the predominant sialomucin pattern was seen mainly in patients with dysplasia. It may, therefore, indicate patients at high risk of malignancy.