OBJECTIVE: Malnutrition during hospital stay has been consistently reported since the 1970s. In addition to the clinical implications malnutrition may have, it also leads to increased length of stay and drug usage due to complications. We report from a large observational database of 26 Belgian hospitals to document the potential economic impact of hospital malnutrition. METHODS: As a legal requirement, Belgian hospitals must register case-mix data for each inpatient stay in a minimum basic data set (MBDS). We extracted exhaustive and anonymous stay data from 26 hospitals (2nd semester, 2003) and identified malnourished patients as patients for whom ICD9-CM codes for “underweight” (783.22) or “severe weight loss” (783.21) were recorded. A matched analysis on APR-DRG, age and gender was then performed to compare inpatient pharmaceuticals costs, procedures costs, hotel cost and overall costs between malnourished and normally nourished patients. RESULTS: A total of 1032 malnourished patients were identified. Of those, “underweight” or “severe weight loss” was the primary diagnosis for 105 patients and the secondary diagnosis for 927 patients. Therefore, only these 927 patients were included in the matched analysis. In all, 26,067 matched controls were retrieved. The overall mean cost difference per stay between malnourished and normally nourished patients averaged €1152 (95%CI: €870; €1433). Pharmaceuticals, procedures and hotel costs differences averaged €264 (€192; €336), €137 (€113; €161) and €754 (€508; €1000), respectively. The largest mean cost difference was found for APR-DRG 691-Lymphoma & non-acute leukemia: €5117 (€2544: €7691). CONCLUSION: The inpatient cost incurred by malnutrition is substantial and calls for routine pre – or in-hospital nutritional screening and adequate and timely initiation of nutritional support. However, we can not exclude the possibility that only severe malnutrition was reported and recognized through hospital registries. Further studies able to report from the larger spectrum of malnutrition are strongly advocated.