The incidence of atopic and immune diseases has dramatically increased during the second half of the twentieth century. This has been attributed to improved sanitation and hygiene with reduced exposure to infections. The concept of this hygiene hypothesis is not new, and is currently used to explain the increasing incidence of a wide area of diseases. Parasitic helminths are powerful modulators of their host's immune system. It is suggested that the reduced exposure to helminths, due to better hygiene conditions, may predispose to the development of inflammatory bowel diseases. This article reviews the current epidemiological, experimental and clinical data supporting the role of helminths in the hygiene hypothesis in inflammatory bowel diseases.