Colorectal strictures, either benign or malignant, are not uncommon in ulcerative colitis. Fifty nine of 1156 ulcerative colitis patients (5%) admitted to this hospital between 1959 and 1983 developed 70 separate colorectal strictures. Seventeen of the 70 strictures (24%) proved to be malignant and the other 53 benign. Nine patients developed more than one stricture. Three principal features distinguished the 17 malignant from the 53 benign strictures in this series: (1) appearance late in the course of ulcerative colitis (61% probability of malignancy in strictures that develop after 20 years of disease v 0% probability in those occurring before 10 years); (2) location proximal to the splenic flexure (86% probability of malignancy v 47% in sigmoid, 10% in rectum, and 0% in splenic flexure and descending colon); and (3) symptomatic large bowel obstruction (100% probability of malignancy v only 14% in the absence of obstruction or constipation). Moreover, cancer associated with strictures tends to be more advanced (76% stage D, 24% A and B) than that which does not produce strictures (18% stage D, 59% A and B).