Objectives: To investigate total daily energy expenditure in chronic obstructive pulmonary disease (COPD) patients during a rehabilitation programme. Design: Observational study involving a case and a control group. Subjects: Ten COPD patients (six with body mass index (BMI) < 18.5 kg=m 2 and four with BMI > 18.5 kg=m 2 ) were evaluated for their energy expenditure profile. Four additional healthy age-matched volunteers were also included for methodology evaluation.Interventions: Measurements of total daily energy expenditure (TEE), resting energy expenditure (REE) and diet-induced thermogenesis (DIT) and energy intake were undertaken by indirect calorimetry and bicarbonate -urea methods and dietary records. Results: REE in COPD patients was not significantly different from that predicted by the Harris -Benedict equation. Before the exercise day the mean TEE was 1508 kcal=day and physical activity level (PAL as calculated by TEE=REE) was 1.52. On the exercise day the TEE increased to 1568 kcal=day and PAL was 1.60, but neither of these changes were significant. The energy cost of increased physical activity during rehabilitation exercise was estimated to be 191 kcal=day. No significant change was found in DIT between the two patient groups. However, overall energy balances were found to be negative ( 7 363 kcal=day). Conclusion: The rehabilitation programme did not cause a significant energy demand in COPD patients. TEE in COPD patients was not greater than in free-living healthy subjects. Patients, who were underweight, did not have a higher TEE than patients with normal weight. This suggested that malnutrition in COPD patients was not due to an increased energy expenditure. On the other hand, a significant negative energy balance due to insufficient energy intake was found in seven out of 10 patients. Sponsorship: The project was inpart supported by the Bristol Myers Squibb Unrestricted Nutrition Grant.