Following the very short course of a disease with watery diarrhea, fever, nausea, meteorism and a severe feeling of general illness, a 22-year-old patient was diagnosed as having a toxic megacolon, and a subtotal colectomy was carried out. The postoperative progression was uncomplicated and the patient recovered quickly. The examination of the operation specimen revealed a serious ulcerous colitis with relative omission of the rectum and the distal sigmoid colon. After critical evaluation of the histological findings, it was judged to be a fulminant Crohn's colitis and, for the purposes of differential diagnosis, differentiated from ulcerative colitis and colitis indeterminate. The formal pathogenesis of the inflammatory-ulcerous processes is discussed, in particular with regard to the activation of the macrophages and the very short anamnesis in a clinically established primary manifestation of the disease.