Summary:The purpose of this investigation was to assess changes in total energy expenditure (TEE), body weight (BW) and body composition following a peripheral blood stem cell transplant and following participation in a 3-month duration, moderate-intensity, mixed-type exercise programme. The doubly labelled and singly labelled water methods were used to measure TEE and total body water (TBW). Body weight and TBW were then used to calculate percentage body fat (%BF), and fat and fatfree mass (FFM). TEE and body composition measures were assessed pretransplant (PI), immediately posttransplant (PII) and 3 months post-PII (PIII). Following PII, 12 patients were divided equally into a control group (CG) or exercise intervention group (EG). While there was no change in TEE between pre-and post-transplant, BW (Po0.01) and FFM (Po0.05) significantly decreased during the same period. Participation in the exercise programme led to increases in TEE to levels that were both higher than pre-and post-transplant measures (Po0.01). By PIII, the exercising patients also showed gains in FFM (Po0.01) in association with a reduction in %BF (Po0.05). Exercise has a functionally important role in preserving and increasing skeletal mass in the rehabilitation phase of cancer patients. Bone Marrow Transplantation (2003) 31, 331-338. doi:10.1038/sj.bmt.1703867 Keywords: energy expenditure; body composition exercise; PBST It is well recognised that if total energy expenditure (TEE) exceeds energy intake (EI), a reduction in body weight (BW) will result. A decrease in BW following transplant for cancer is a common side effect, 1 particularly for patients undergoing a peripheral blood stem cell transplantation (PBST). Elevated metabolic demands and impaired protein-sparing mechanisms, in conjunction with reduced energy intake, inactivity, and specific chemotherapeutic and steroid treatment regimens, may be contributing reasons for the negative energy balance, and therefore weight loss observed in some cancer patients. 2,3 However, the first step in learning the aetiology of weight changes following cancer treatment is to measure either one or both sides of the energy equation, TEE and total energy intake.Total energy expenditure comprises resting energy expenditure (REE), thermogenesis, physical activity and the energy cost of growth. 4 Of the limited studies available that have assessed TEE in patients with cancer, TEE has been calculated by measuring REE and the energy cost of physical activity. [5][6][7] These studies usually maintain the assumption that thermogenesis accounts for approximately 10% of TEE, while the energy cost of growth is negligible. However, the former assumption may be violated because of the adverse effects that symptoms commonly experienced by cancer patients, such as nausea, vomiting and pain, have on appetite and food intake. Many chemotherapeutic agents, including dactinomycin, bleomycin, cyclophosphamide, 5-fluorouracil, methotrexate and vincristine, all have the potential to reduce food intake by causing nause...