There is some evidence that environmental factors cause ulcerative colitis. The incidence of ulcerative colitis has increased dramatically between World War II and the 1980s.1 More recent data show that, in several areas, ulcerative colitis incidence decreases.2 3 Such large fluctuations are not consistent with a purely genetic disease, although the increased incidence could be attributed to a better recognition of the disease (coinciding with progress in endoscopy) and the recent decline, to a diagnostic transfer from ulcerative colitis to Crohn’s disease. Migrant studies have shown that ulcerative colitis incidence is at least as high in subjects originating from South Asia living in UK than in native British subjects,4 and has increased recently,5 thus suggesting that the British way of life increases the incidence of ulcerative colitis in migrants from Asia. Another evidence for the role of environment in ulcerative colitis comes from twin studies: disease concordance in monozygotic twins is only 19% in ulcerative colitis, as opposed to 50% in Crohn’s disease.6