Case detection In Spain, health care is performed mostly by the public health services. According to recent data, approximately 15% of the Spanish population has private health insurance (Informe Sanidad Privada: Aportando Valor. Análisis de situación 2014 (Report on Private Health Care: Providing Value. 2014 Situation Analysis); available at: https://www.fundacionidis.com/wp-content/informes/informe_analisis situac_2014_0.pdf). In addition, of those persons having private health insurance, only about 15% make exclusive use of it. For these reasons and taking into account the specific characteristics of IBD, the risk of underestimating the incidence of IBD considering only cases seen in public health centers would be of little relevance. Therefore, this study will be conducted at centers providing public health care within the National Health System.