BACKGROUND & AIMS: Undernutrition is a common and aggravating problem in people suffering from various chronic diseases as well as a source of material costs. The aim of this study was to investigate the prevalence of disease-related undernutrition among adults in Croatia in the year 2012, as well as the cost of undernutrition associated with tumour cachexia, chronic pancreatitis, inflammatory bowel disease, hepatic encephalopathy, chronic obstructive pulmonary disease, chronic renal insufficiency requiring dialysis, cerebrovascular insult, pressure ulcers, and femoral fractures in the elderly. ----- METHODS: A cost-of-illness analysis was conducted, including direct costs only. The study employed the dominant cost-of-illness method, which restricts the summation of medical expenditure to the disease of interest. The bottom-up, prevalence-based approach was used. The budget holder perspective was employed, and data sources include the list of reimbursed drugs, clinical opinions, and literature. ----- RESULTS: The prevalence of disease-related undernutrition in people over the age of 20 in Croatia in the year 2012 was estimated to be 33.7/1000. The total cost of adult malnutrition for selected diagnoses was 97.35 million EUR, accounting for 3.38% of the total Croatian national health care budget. The largest share was used for medications (43%), followed by 34% for hospitalizations, 13% for community health nursing, while parenteral and enteral nutrition contributed with 6% and 1% respectively. The average cost per patient was estimated at 1640.48 EUR. ----- CONCLUSIONS: The cost of malnutrition for the selected diagnoses in Croatia was substantial. These health costs will increase due to population ageing, which calls for undernutrition screening in people at risk as well as for effective approaches in nutrition supplementation.