Abstract Objectives In Luxembourg, the frequency of CT and MRI examinations per inhabitant is among the highest in Europe. A national audit was conducted to evaluate the appropriateness of CT and MRI examinations according to the national referral guidelines for medical imaging. Methods Three hundred and eighty-eight CT and 330 MRI requests corresponding to already performed examinations were provided by all radiology departments in Luxembourg. Four external radiologists evaluated the clinical elements for justification present in each request. They consensually assessed the appropriateness of each requested examination with regard to the national referral guidelines and their clinical experience. Results The appropriateness rate (AR) was higher for MRI requests than for CT requests (79% vs. 61%; p < 0.001). AR was higher for requests referred by medical specialists rather than by general practitioners, both for CT requests (70% vs. 37%; p < 0.001) and MRI requests (83% vs. 64%; p = 0.002). For CT, AR was higher when the requests concerned paediatric rather than adult patients (82% vs. 58%; p < 0.001), when the radiology departments were equipped with both CT and MRI units rather than with only CT units (65% vs. 47%, p = 0.004) and when the requests concerned head-neck (79%), chest (77%) and chest-abdominal-pelvic (81%) areas rather than spinal (28%), extremity (51%) and abdominal-pelvic (63%) areas (p < 0.001). Conclusions The appropriateness of CT and MRI in Luxembourg is not satisfactory and collective efforts to improve should be continued. The focus should be on general practitioners and on spinal CT examinations.