This study calculates the social costs and benefits of dietetics for malnourished patients with gastro-intestinal or lung cancer, patients with head and neck cancer and elderly hospital patients. For every euro spent on dietetics for these patients the benefit to society is 3.08-22.60 euros (gastro-intestinal or lung cancer), 2.40-4.50 euros (head and neck cancer) and 1.20-1.90 euros (malnourished elderly hospital patients). If all malnourished patients were to be treated the total annual benefit would be 4-42 million euros (gastro-intestinal or lung cancer), 1.5-3.8 million euros (head and neck cancer) and 15-78 million euros (elderly hospital patients).