For financial reimbursement, Belgian hospitals need to comply with compulsory registration of the Minimal Hospital Data (MHD). These comprise various data on all hospital admissions at patient level. Our ultimate goal is to query this comprehensive repository to answer various health care related questions in the area of rare diseases (80% of which are genetic diseases). Unfortunately, such analyses are hampered by lack of a suitable codification/classification system in the hospital information systems. Although efforts are being made to introduce the ORPHA rare disease codes, diagnostic information in the MHD was coded with ICD-9-CM. Since January 2015, ICD-10-CM is in use .In order to investigate the problem of rare disease coding more thoroughly and to enable the exploitation of historical data, we aimed to identify all codes of interest in the ICD-9-CM and ICD-10-CM. Mapping and linearization of the ORPHA codes to ICD-10-CM codes is done by Orphanet (INSERM) and available at orphadata.org. ICD-9-CM/ ICD-10-CM conversion tables were created by the FPS. By standardizing and combining these lists we distilled a final set of ICD9-CM codes for rare diseases of equivalent, broader as well as narrower terms.We conclude that for rare diseases, the transition to ICD-10-CM was indispensable due to a higher degree of granularity. We will discuss the mapping methodology, its accuracy and results. In addition, we will illustrate the value of MHD analyses for identification of rare diseases by presenting information derived from MHD for a representative rare disease.