The purpose of this study is to compare five commonly used malnutrition screening tools against an acknowledged definition of malnutrition in one hospital inpatient sample. Early identification and intervention of malnutrition in hospital patients may prevent later complications. Several screening tools have reported their diagnostic accuracy, but the criterion validity of these tools is unknown. A cross sectional study. We compared quick-and easy screening tools [Malnutrition Screening Tool (MST), Short Nutritional Assessment Questionnaire (SNAQ) and Mini-Nutritional Assessment Short Form (MNA-SF)] and more comprehensive malnutrition screening tools [Malnutrition Universal Screening Tool (MUST) and Nutritional Risk Screening 2002 (NRS-2002)] to an acknowledged definition of malnutrition (including low Body Mass Index and unintentional weight loss) in one sample of 275 adult hospital inpatients. Sensitivity, specificity, positive predictive value and negative predictive value were determined. A sensitivity and specificity of ≥ 70% was set as a prerequisite for adequate performance of a screening tool. According to the acknowledged definition of malnutrition 5% of patients were at moderate risk of malnutrition and 25% were at severe risk. The comprehensive malnutrition screening tools (MUST, NRS-2002) and the quick-and-easy malnutrition screening tools (MST and SNAQ) showed sensitivities and specificities of ≥70%. However, 47% of data were missing on the MUST questionnaire and 41% were missing on MNA-SF. The MNA-SF showed excellent sensitivity, but poor specificity for the older subpopulation. The quick-and-easy malnutrition screening tools (MST and SNAQ) are suitable for use in an hospital inpatient setting. They performed as well as the comprehensive malnutrition screening tools (MUST and NRS-2002) on criterion validity. However, MUST was found to be less applicable due to the high rate of missing values. The MNA-SF appeared to be not useful because of it low specificity. Insight in what is the most valid and practical nutritional screening tool to use in hospital practice will increase effective recognition and treatment of malnutrition. © 2011 Blackwell Publishing Ltd.