Nutritional status (NS) monitoring is an essential step of the nutrition care process. To assess changes in NS throughout hospitalisation and its ability to predict clinical outcomes, a prospective cohort study with patients over 18 years of age was conducted. The Subjective Global Assessment (SGA) was performed within 48 h of admission and 7 d later. For each patient, decline in NS was assessed by two different methods: changes in SGA category and severe weight loss alone (≥2 % during the first week of hospitalisation). Patients were followed up until discharge to assess length of hospital stay (LOS) and in-hospital mortality and contacted 6 months post-discharge to assess hospital readmission and death. Out of the 601 patients assessed at admission, 299 remained hospitalised for at least 7 d; of those, 16·1 % had a decline in SGA category and 22·8 % had severe weight loss alone. In multivariable analysis, decline in SGA category was associated with 2-fold (95 % CI 1·06, 4·21) increased odds of prolonged LOS and 3·6 (95 % CI 1·05, 12·26) increased odds of hospital readmission at 6 months. Severe weight loss alone was associated with 2·5-increased odds (95 % CI 1·40, 4·64) of prolonged LOS. In conclusion, deterioration of NS was more often identified by severe weight loss than by decline in SGA category. While both methods were associated with prolonged LOS, only changes in the SGA predicted hospital readmission. These findings reinforce the importance of nutritional monitoring and provide guidance for further research to prevent short-term NS deterioration from being left undetected.