Twelve new cases of acute toxic dilatation of the colon in Crohn's colitis treated by total colectomy and ileorectal anastomosis are reported, and criteria for making the diagnosis are given.The complication has occurred in 6% of patients diagnosed as having Crohn's disease of the colon treated at the Gordon Hospital since 1948. The patients have been predominantly young adult females, with a short preoperative history of disease. The sigmoidoscopic appearance of the rectum preoperatively has been no indication of the likelihood of recurrent disease at the anastomosis. Only four patients still have successfully functioning ileorectal anastomoses and four patients have died from the effects of the disease.