BACKGROUND Brazilian population has passed for a process of demographic transition throughout latest years, characterized for the increase of the elderly population. Malnutrition is a serious problem to frail elderly. OBJECTIVE The objective of this study was o evaluate the risk of malnutrition among institutionalized elderly resident in municipal shelters in the city of Rio de Janeiro, Brazil, using the tool Mini Nutritional Assessment (MNA). METHODS 344 institutionalized elderly aged over 60 years old were tested in a cross-sectional study using MNA. This tool classifies the nutricional status of the elderly in three groups: malnutrition (score < 17), risk of malnutrition (score 17 - 23,5) and well-nourished (score > = 24). Anthropometric measurements such as calf circumference (CC), mid-arm circumference (MAC) and Body mass index (BMI) were also evaluated. The variables were evaluated using the chi-square or ANOVA test. To correlate it was used Pearson's Correlation Coefficient (r). RESULTS Mean age were 75.4 (+- 9.4) years old. Most of the elderly were female gender (59.6%). According to MNA 8.3% were with malnutrition, 55.6% at risk of malnutrition and 36.1% well-nourished. BMI classified 10.0% of the elderly as underweight. CC classified 10.0 % of them as inadequate in muscular mass. MNA was well correlated to BMI (r=0.412 p=0.000), age (r=-0.124 p=0.031), CC (r=0.399 p = 0.000) and MAC (r=0.391 p=0.000). CONCLUSIONS Risk of malnutrition was high among the institutionalized elderly from public shelters in Rio de Janeiro - Brazil. MNA is a useful diagnostic tool for the identification on the frail elderly at risk of malnutrition.