Leptin is secreted by adipose tissue and acts upon receptors located in the hypothalamus to modify energy balance. Investigations of the relationship between leptin and physical activity energy expenditure (PAEE) at population level are scarce. The majority of studies addressing this topic are limited by their measurement of PAEE (i.e. questionnaires or ecological comparisons between rural and urban ethnic groups). To our knowledge, no studies have directly examined the relationship of objectively assessed PAEE and leptin in a large free-living population-based cohort. Therefore, we measured fasting plasma leptin and insulin concentrations, cardiorespiratory fitness (O(2max.pred)), PAEE, and body composition in 758 Caucasian people (aged 40-65 yr). In sex-combined multiple regression analyses, leptin was significantly associated with PAEE (beta = -0.19, P = 0.0027), but not with O(2max.pred) (beta = -0.0002, p = NS). The association between PAEE and leptin was significant in men when adjusted for percentage of body fat (beta = -0.28, P = 0.004) but not women (beta = -0.12, P = 0.18) but was significant in both men and women when adjusted for body mass index (men: beta = -0.28, P = 0.005; women: beta = -0.23, P = 0.01; combined: beta = -0.26, P = 0.00008). These data suggest the existence in this population of an independent inverse association between PAEE and fasting plasma leptin level.