Weight loss occurs in about a third or more disabled patients with chronic obstructive pulmonary disease (COPD), and appears to be a poor prognostic factor. As such, it correlates only weakly with FEV1, transfer factor and other measures of respiratory physiology and is probably, to a certain extent, independent of them. Recent studies of basal metabolic rate (BMR) in COPD using steady-state, non-invasive calorimetry, have shown it to be elevated by 10-20% in up to 40% of such patients. It is likely that this represents true hypermetabolism per kilogram of fat free mass. An elevated BMR cannot be predicted from combinations of detailed lung function tests or arterial gases, as patients with similar physiology have differing BMRs. Thus, although an increased work of breathing is the probable explanation for some of the increase, other factors such as cytokines or possibly drug therapy almost certainly contribute. Muscle loss in weight-losing COPD appears to involve both type I and type II fibres, because of a combination of reduced calorie intake and disuse atrophy. Respiratory muscles share this fibre loss. Review of the controlled studies on nutritional supplementation in COPD suggests that an energy increase of about 30% is needed to achieve substantial weight gain and improve exercise tolerance. Fat-rich supplements have some theoretical advantages. Further work is needed particularly with regard to the determinants of the increased BMR in COPD, and the effect of longer term nutritional supplements on prognosis.