Background: Evidence regarding the double burden of malnutrition (DBM) among hospitalized patients in Vietnam remains limited. This study examined nutritional status at admission and its association with length of hospital stay among adults in Hanoi. Methods: This prospective observational cohort study was conducted in eight public hospitals in Hanoi between September 2018 and November 2019. Adults aged 18-60 years were assessed within 48 hours of admission using interviews, physical examination, anthropometric measurements, and medical records. Nutritional status was classified using the Subjective Global Assessment (SGA) and body mass index (BMI): undernourished (SGA-B or SGA-C or BMI