We examined the distribution of colonic mucosal mast cells in 25 patients with active ulcerative colitis, with a clear line of demarcation separating active inflammation from normal mucosa. Biopsies, at least one adjacent to the line of demarcation, one in inflamed mucosa, and one above were obtained during colonoscopy. Eight patients had elevated mast cells throughout the colon, and 12 had increased numbers at the line of demarcation of disease. Mean numbers of mast cells from these patients were 6.3 (+/- 2.1 SD) in active inflammation, 19.5 (+/- 7.1 SD) at the line of demarcation, and 15.8 (+/- 8.4 SD) in normal mucosa. Histologic inflammation decreased as mast cells increased. The accumulation of mast cells at the visible line of demarcation between normal and abnormal mucosa suggests mast cells play a critical role in either accelerating the process of inflammation or in suppressing continued extension of the disease.