There is a body of clinical, epidemiological, biologic, histological and therapeutic data suggesting that type I hypersensitivity plays a role in the pathogenesis and maintenance of ulcerative colitis. Contradictory evidence from different studies on the pathogenic mechanisms may simply mean that there is not one but several types of ulcerative colitis. Chronic inflammation of the intestine would cover a heterogeneous group of conditions. Genetic susceptibility controlling one of more anomalies of the immune system would be triggered by external factors such as respiratory or food allergies, viral or bacterial infections or other factors including smoking or stress. The wide range of factors involved would explain the variety of findings reported by different groups searching for a single pathogenic mechanism. Finally, as emphasized by other authors, screening for subgroups of patients with allergy among the ulcerative colitis population would be useful in adapting treatment and developing a more specific therapeutic strategy not only for acute phases but perhaps also for preventive treatment.