Aim: The aim of this study was to investigate factors that may affect the evolution of the caloric prescription in critically ill patients. Local: Intensive care unit patients. Patients: 60 patients (33 M and 27 F); median age = 49 (15-93) y were followed prospectively. They were divided in three groups according to the diagnostic: a) trauma (n = 20); b) surgical (n = 22), and 3) medical treatment (n = 18). Forty-and-one (68.3%) patients received enteral nutrition (EN), 17 (28.3%) parenteral nutrition (TPN), and 2 (3.4%) TNP and EN. Nutritional status was graded B or C by global subjective evaluation. Methods: Endpoints of the study were the time to begin the nutritional support, success or failure of the caloric prescription, and the evolution of the planned caloric prescription. The caloric evolution was considered as success if the prescription for the patient attained: a) 25% of the caloric requirements on the 1st day; b) 50% until the 3rd day; c) 75% until the 6th day; and e) 100% until the 10th day of the beginning of the support. Results: In 54 (90%) patients, the nutritional support has begun until 48h after admission and in 73.3% (44 patients), until the first 24 hours. EN was most prescribed for both trauma and medical patients while NPT was most used for surgical patients (p < 0.01). Success in caloric prescription was obtained in 73.3% (44) of the patients. There was no statistical difference for the success on the evolution of the prescription related to sex, age, diagnostic group, albumin level, type of support, mortality, use of fiber or glutamine. Success was attained earlier in patients without (median = 3.8 [95%CI, 5.7-16.7] days) than with (11.2 [95%CI, 5.7-16.7] days; p < 0.01) mechanical ventilation. Conclusions: Early nutritional support and success on the evolution of the caloric prescription can be accomplished in most critically ill patients. Evolution of the caloric prescription was slower in mechanical ventilated patients.