The prevalence of undernutrition in hospital populations is known to be high. The presence of malnutrition is associated with depression, infections, sarcopaenia, falls, fractures, reduced autonomy and increased mortality. This study specifically examined the prevalence of malnutrition in patients aged 65 or older at the time of admission as determined by the Mini Nutritional Assessment (MNA) which has been a frequently used nutritional risk screening tools in detecting undernutrition in old people. This study was done for one hundred eight hospitalized geriatric patients in Seoul National University Bundang Hospital, Seoul, Korea. On admission baseline history, anthropometrics measurements, laboratory data and nutritional status by MNA were assessed. Length of hospital stay was obtained by reviewing medical charts. We used one-way analysis of variance to compare the differences in variables. Spearman's rank correlation coefficients were calculated for associations between MNA and variables. On admission, 22.3% of patients were malnourished and 40.7% were at risk of malnutrition according to the MNA. Percent of ideal body weight, anthropometrics data, albumin, and hemoglobin were lower in the malnourished patients (p < 0.05). The malnourished patients stayed in the hospital 7.3 days longer, as compared with well nourished patients (p < 0.05). Percent of ideal body weight, albumin, hemoglobin and total cholesterol were correlated inversely with nutritional status according to MNA (p < 0.05). MNA can be used for nutritional assessment in Korean old people, because MNA significantly correlated with other nutritional assessment parameters, such as, anthropometric and laboratory data in hospitalized geriatric patients. The high prevalence of malnutrition in the elderly was observed and the presence of malnutrition on admission predicted a significant increase in the length of hospital stay in this study. Therefore further studies are needed to determine whether nutritional interventions in old people with low MNA scores can improve clinical outcomes during the hospital course.