Obesity, prevalent in industrialised societies, is most usefully categorised by means of the body mass index (BMI-weight/height2). A body mass index of greater than 25 is associated with increasingly poor prognosis. Weight reduction has been shown to be beneficial with respect to both mortality and morbidity. Excess weight results from an imbalance between energy input and expenditure in favour of the former. Weight reduction may be promoted by reducing energy input and/or stimulating expenditure. It is tempting to postulate that inactivity may be a factor in both the development and subsequent continuation of obesity via an effect on energy intake, fat-free mass or energy expenditure. Although available data are by no means conclusive, the majority of evidence suggests that obesity is not associated with either reduced activity or energy expenditure. Likewise, exercise appears not to promote a change in body composition in favour of lean body mass or have a prolonged thermogenic effect beyond the duration of the activity. Exercise alone appears largely ineffective regarding weight loss and almost certainly has to be coupled with calorie reduction. It must be remembered that the exercise tolerance in the severely obese (BMI greater than 40) is very poor. Such individuals must be closely supervised during a specifically graded programme. What is also apparent is the high drop-out rate of individuals recruited into exercise programmes. In those who voluntarily engage in physical activity, the incidence of ischaemic heart disease may be reduced, which may or may not be related to a direct exercise effect on known cardiovascular risk factors. Cigarette smoking is usually less common and general life-style may be more prudent. Whether exercise in obese subjects could have a similar effect remains unknown. Although much of the data on exercise in general and on obesity in particular are negative, it appears unwise to adopt a totally nihilistic approach. Increased physical activity should be encouraged as it is possible that the discipline involved in regularly undertaking such activity may be more conducive to weight loss, a feeling of well-being and fitness and a general change of life-style for the better. Long term it may also afford additional benefit by reducing liability to ischaemic heart disease.