The white blood cell (WBC) count is correlated to the amount of body fat in humans, but the mechanism for this association is unknown. Leptin, a 16 kD protein produced in adipocytes, circulates in humans in direct proportion to the amount and percentage of body fat. Recent evidence suggests that leptin and the leptin receptor are part of a novel pathway which stimulates haemopoiesis. This study was designed to test whether fasting plasma leptin concentrations contribute to the relationship between the WBC count and body fat. 117 Pima Indians with a wide range in body composition were studied. The WBC count was positively correlated with percentage of body fat (r = 0.44, P = 0.0001) and fasting plasma leptin concentration (r = 0.38, P = 0.0001). In multiple regression analyses, age, gender and percent body fat were significant independent determinants of the WBC count. After controlling for age and gender, percent body fat accounted for 23% of the variance in the WBC count (partial r = 0.48, P = 0.0001). In similar models which also included plasma leptin concentration, percent body fat remained significantly related to the WBC count. but only accounted for 7% of its variance (partial r = 0.27, P = 0.003). Based on these results, and the demonstration that leptin directly effects the stimulation of proliferation of haemopoietic stem cells in vitro, we hypothesize that the relation of the WBC count to percent body fat may be mediated, in part, through the effect of leptin.