Malnutrition is considered an important clinical problem in geriatric hospitalized patients, but no standard diagnostic criteria are available. The aim of this study was to investigate the effect of 7 different sets of diagnostic criteria (6 in hospitalized elderly and 1 in healthy elderly) from nutritional research studies on the prevalence of malnutrition in 151 elderly patients (49 men, 102 women, mean age 82.8 years) consecutively admitted to an acute geriatric ward. Nutritional status was assessed using anthropometric (body mass index, triceps skinfold thickness, mid arm circumference, arm muscle circumference), and laboratory (serum albumin) parameters, and 1 nutritional questionnaire (Mini Nutritional Assessment). The proportion of patients with malnutrition varied between 6.5 and 85%. It is important to assess the nutritional status in elderly patients because the prevalence of malnutrition in hospitalized elderly patients is substantial whatever criteria are used. However, the criteria for the diagnosis can differ by a factor of more than 10 in the number of patients classified as malnourished. A major goal for further research on malnutrition is to define more standardized and validated criteria in order to make comparisons of prevalence and outcome more meaningful, and ensure that all those who will benefit from a nutritional intervention are identified and treated.