Objective: The aim of this study was to determine the validity and reliability of the Malnutrition Universal Screening Tool (MUST) and the Malnutrition Screening Tool (MST) in hospital inpatients with renal disease. Design: A cross-sectional and longitudinal study. Setting: The study took place on 3 renal inpatient wards in a tertiary hospital in south London. Patients: A total of 276 participants were recruited. Intervention: Not applicable. Main Outcome Measure: Concurrent validity was assessed by comparing the MUST and MST tools completed by nursing staff with the subjective global assessment tool completed by dietetic staff. Predictive validity was evaluated by assessing the association between malnutrition and length of hospital stay. Mid-upper arm circumference and bioelectrical impedance spectroscopy were used to assess construct validity. In the reliability study, the MUST and MST tools were repeated on the same day by nursing staff. Objective: MUST had a sensitivity of 53.8% (95% confidence interval [CI], 46.6% to 60.0%) and a specificity of 78.3% (95% CI, 70.1% to 85.2%), and MST had a sensitivity of 48.7% (95% CI, 41.7% to 54.0%) and a specificity of 85.5% (95% CI, 77.9 to 91.3) when compared with subjective global assessment. Risk of malnutrition as identified by MUST but not the MST tools had a significantly longer length of hospital stay (P = .038 and .061). Both MUST and MST tools identified patients at risk of malnutrition had a significantly lower mid-upper arm circumference (P = .005 and P = .029, respectively) and percent fat mass (P = .023 and P = .052, respectively). Reliability assessed by kappa was 0.58 for MUST (95% CI, 0.20 to 0.80) and 0.33 for MST (95% CI, -0.03 to 0.54). Conclusions: The MUST and MST nutrition tools are not sensitive enough to identify all of the malnourished renal inpatients, despite being fairly reliable and related to other nutrition status markers.