OBJECTIVES: Patients undergoing hematopoietic stem cell transplantation may present with metabolic alterations that can have an effect on their energy expenditure and nutritional status. This project aimed to compare the pre- and posttransplant energy expenditures of patients undergoing hematopoietic stem cell transplantation as well as related factors. METHODS: This prospective study was conducted at a single center. Patients, undergoing autograft or allograft, were evaluated before transplantation and on the 10th and 17th d posttransplantation. Energy expenditure was measured by indirect calorimetry. Diet intake was assessed by a 24-h dietary recall. Infectious and noninfectious complications were analyzed between days 1 to 10 after transplantation and days 11 to 17 after transplantation. Paired model analyses were carried out to identify the pretransplantation and posttransplantation periods. RESULTS: Twenty patients were evaluated with a mean age of 45.6 ± 17.2 y; a majority were male sex (65%), and the most frequent diagnoses were chronic myeloid leukemia (25%) and multiple myeloma (25%). Energy expenditure increased by 15% posttransplantation, and the energy requirement per kilogram of weight was 23 kcal/kg at day 10 after transplantation. Throughout the posttransplantation period, 45% of the patients required nutritional therapy. Negative energy and negative protein balance were observed at all analyzed times. Phase angle (P = 0.018), fever (P = 0.014), mucositis grades I to II (P = 0.018), and the total number of infectious and noninfectious events (P = 0.043) were associated with an increase in energy expenditure at day 10 after transplantation. CONCLUSIONS: Energy expenditure increased after transplantation compared with pretransplantation in 50% of patients. Phase angle, fever, grades I to II mucositis, and infectious and noninfectious events were associated with increased energy expenditure at day 10 after transplantation.