The current guidelines for physical activity are based on the prevention of cardiovascular disease. In this article the magnitude and type of physical activity required to prevent unhealthy weight gain are assessed. Five categories of analyses are considered, ranging from the most rigorous analyses (based on D2O18 measures of energy expenditure) to socio-ecological associations. To standardize the approach, published work on the extent of exercise was expressed as a physical activity level (PAL), i.e. the ratio of total expenditure to the measured or estimated basal metabolic rate. D2O18, direct monitoring and measurements of activity patterns and detailed prospective studies of substantial population groups all suggest that a PAL of > or = 1.8 is required to limit the proportion of overweight and obese adult men. Data on women are more difficult to interpret because women are less active and the relationship with physical activity is usually less clear. Post-obese women with a PAL of >1.75 do not regain weight and other data are consistent with the need for a PAL of > or = 1.8. The analyses in both sexes are based predominantly on adults living in a Western society with the ready availability of energy-dense foods. Vigorous activity is more clearly linked to weight stability, allows a higher intensity of exercise for general activities and shortens the time needed for achieving a PAL of 1.8. This activity level is equivalent to an additional 60-90 min of brisk walking in adults who normally undertake only modest exercise. These demands are greater than the current suggested levels for cardiovascular benefit and imply the need for different environmental policies, rather than health education policies, if societies are to become generally more active and avoid unhealthy weight gain.