UNLABELLED: The Refeeding Syndrome is conformed by a series of clinical manifestations related to electrolytic alterations associated with the restarting of the nutritive contribution both enteral and parenteral. AIM: To detect the Refeeding Syndrome incidence in malnourished patients who required nutritional, enteral or endovenous support and its relationship with mortality. MATERIAL AND METHODS: A cohort study was performed in the service of Nutritional Support of the IMSS (Social Security Mexican Institute) Specialties Hospital CMN León, from June 1995 to May 1996. All patients with mild and severe malnutrition were included, they received endovenous or enteral nutritious support for more than 7 days, without presenting previous electrolytic unbalance. Serum potassium, phosphorous, and magnesium levels were determined before starting the nutritious support and also on the 3rd, 7th, and 10th days. Descriptive statistics, Student's t and Z test were used, with a 5% significance level. RESULTS: 148 patients with total nutritional support, 23 (16%) of them with restrained malnutrition and 65 (44%) with severe deficit. 54 men and 34 women with an average age of 51.6 +/- 19.4 years. Nineteen patients were eliminated due to a nutrition period of less than 7 days, and other 19 were also eliminated for presenting electrolytic alterations before the nutritive support started. An incidence of 48% of electrolytic alterations compatible with the refeeding syndrome was the result in the remaining 50 patients. The alterations were: hypomagnesemia 13/24, hypokalemia 12/24 and hypophosphatemia 4/24; in 55% of the cases the syndrome appeared at the third day of administration. Hospital sojourn of patients with the syndrome was 26.7 +/- 18 days vs 15.3 +/- 7 (p < 0.05) of those who did not present it. 15 patients died, 5 of them had electrolytic alterations before nutrition, 7 (29%) with refeeding syndrome and 3 (12%) did not presented it (p = 0.059). CONCLUSIONS: Refeeding Syndrome is a frequent entity in malnourished patients submitted to enteral or parenteral nutrition; at least in this study it was of 48%; its presence was followed by a longer hospital stay and a higher mortality rate.