Caloric expenditure due to physical activity is a potentially important and largely neglected factor in human obesity. The present study indicates that decreased physical activity may play a role in the production and maintenance of the obesity of at least some persons. Obese women appear to be far less active than nonobese women, and this difference in activity is paralleled by differences in attitudes toward activity. An investigation of the reasons for this decreased activity indicates that physical activity in man is determined by a variety of influences: biological, social, and emotional. One of the most important and most common of these influences is the series of events associated with a depressive reaction pattern. Depression, thus, may not be a purely incidental occurrence in obese persons. It may be one of the main reasons why they are obese.