Undernutrition is commonly associated with chronic disease in children and the elderly. Overnutrition is also, but less commonly, associated with chronic illness. In most diseases malnutrition arises because energy intake does not match energy output. Traditionally, the focus of research has been on abnormalities in energy expenditure, in the belief that these factors were the main determinants of energy imbalance. Recent studies using the doubly-labelled-water method to measure total energy expenditure, combined with more complex study design, have suggested an alternative conclusion. In many chronic diseases patient behaviour, and particularly energy intake, is responsible for energy imbalance and malnutrition. Energy balance studies have therefore provided a useful foundation for the design of strategies aimed at preventing or managing chronic malnutrition. However, modifying patient behaviour is an ambitious undertaking which may not be within the scope of existing clinical nutrition services. A number of non-traditional models of managing chronic malnutrition in children and the elderly are promising. Increasing recognition of the value of systematic review will also provide improved strategies for prevention and management of chronic malnutrition.