Introduction: The presence of hypophosphatemia is strongly related to the occurrence of refeeding syndrome in critically ill patients, in which one of the main risk groups is the elderly population. Objectives: To assess the prevalence of hypophosphatemia and the risk of refeeding syndrome in elderly patients admitted to an intensive care unit. Methods: Prospective observational study carried out in an intensive care unit with elderly patients of both genders using enteral nutritional therapy. Demographic, clinical and biochemical data were collected, and nutritional screening and assessment were performed. The energy and nutrient requirements were calculated and a cutoff point of 90% was adopted to establish the adequacy of the caloric supply. To assess the risk and occurrence of refeeding syndrome, the criteria proposed by the NICE group were used. Statistical analyses were performed using the SPSS 13.0 program, with a 95% confidence interval (CI). Results: 44 patients were studied, of which 34.1% were malnourished; 86.4% of patients started enteral nutritional therapy within 48 hours, with 43.2% of caloric adequacy within 72 hours. Hypophosphatemia was found in 9.1% of patients on admission and in 29.5% after starting the diet. Thus, 88.6% of patients had some risk of developing the refeeding syndrome and 40.9% of them manifested the syndrome. Conclusion: A high prevalence of hypophosphatemia was identified after starting nutritional therapy. In addition, the risk of developing refeeding syndrome was high and its manifestation is similar to data found in the literature.