Malnutrition has high prevalence among hospitalized patients but goes unrecognized in many patients. Early detection of malnutrition using an effective screening tool is required. This study aimed to examine the effects of nutritional status determined by the Nutrition Alert Form (NAF) and its individual sections on length of stay (LOS) and hospital costs in hospitalized patients, to investigate their associated factors, and to determine hospital malnutrition prevalence.This retrospective cohort study enrolled 2,906 hospitalized patients aged >=15 years in Ramathibodi Hospital between January and September 2016. At admission, nutritional status was screened using NAF. Nutrition status was defined as: NAF-A (normal/mild malnutrition; scores of 0-5), NAF-B (moderate malnutrition; 6-10), and NAF-C (severe malnutrition; >=11). Information regarding LOS and hospital costs during patients' hospitalization was also collected.The prevalence of malnutrition was 15.3%. After adjusting for age, sex and primary diagnosis, we found significantly longer LOS and higher hospital costs among those with NAF-B and NAF-C, in comparison with patients having NAF-A. The highest increase in LOS was in male patients aged >=60 years with NAF-C. The highest increase in LOS and hospital costs was associated with higher scores for functional capacity.Higher levels of malnutrition screened using the NAF were significantly associated with longer LOS and higher hospital costs. Older adult patients had the highest risk of being malnourished and developing negative consequences. A prospective study of nutritional support by a nutrition care team is underway.