Cachexia is a syndrome and therefore does not have a specific definition. Patients are characterized by the presence of anorexia, early satiety, weight loss, weakness, anaemia and oedema. These features occur to a variable extent in different patients and may change in severity during the course of a patient's illness. The multifactorial origin of cachexia precludes a uniform pathophysiological definition. Taken together these factors have hindered clinical studies both at a fundamental level and in terms of the introduction of effective therapy. The advent of novel therapeutic targets (e.g., ubiquitin-proteasome pathway) and biological response modifiers has opened possibilities for new clinical trials in cachexia. Regulatory authorities feel it is important not only to demonstrate efficacy in terms of patients' nutritional status (e.g., lean body mass) but also functional status (e.g., performance status). This article reviews current methods to assess the latter. Methods focused on measuring physical activity level (e.g., doubly labelled water technique or physical activity meters) promise objective data which can be readily interpreted in terms of clinically meaningful benefit.