Population: Every adult IBD patient, diagnosed in South Limburg after 1991 and permanently residing in South Limburg (the Netherlands), is eligible to participate. The population based nature was reached via a multi-faceted approach; incident cases were prospectively identified through the participating hospitals, and missed patients were retrospectively identified using the nationwide histopathology registry. In 2011, over 3500 patients were included, representing >93% of the IBD population in South Limburg. Adults and partners of included patients serve as controls.