The growing use of CT technology despite the introduction and wider use of other modalities, such as magnetic resonance imaging, raises concerns in radiological protection, especially for children and adolescents. Children are generally more sensitive to the carcinogenic effects of ionizing radiation than adults. In addition, they may receive even higher radiation doses from a CT procedure than an adult. The long-term risk of radiation induced cancer or other health effects following CT scanning has not been directly assessed, although the scientific knowledge so far does not exclude that the ionizing radiation, which is emitted by CT, may cause cancer. No large-scale epidemiological studies of health risks associated with this procedure have been reported so far. The "Epidemiological study to quantify risks for paediatric computerized tomography and to optimise doses" (EPI-CT) was set-up to investigate the relationship between the exposure to ionizing radiation from CT scans in childhood and adolescence and possibly attributable late health effects. Eighteen centres from Belgium, Denmark, Germany, Finland, France, Luxembourg, the Netherlands, Norway, Spain, Sweden and the United Kingdom will cooperate in this project to enrol approximately one million patients. The knowledge gained on current and past CT examination practice will help to propose strategies for further dose reduction.