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 fat-free mass (FFM). TEE and body composition measures were assessed pretransplant (PI), immediately post-transplant (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 (P<0.01) and FFM (P<0.05) 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 (P<0.01). By PIII, the exercising patients also showed gains in FFM (P<0.01) in association with a reduction in %BF (P<0.05). Exercise has a functionally important role in preserving and increasing skeletal mass in the rehabilitation phase of cancer patients.