Abdominal adipose tissue leptin production was determined in vivo by arteriovenous balance in 14 lean and obese men (mean BM127.0 ± 1.9, range 21.4-45.2). Blood samples were taken simultaneously from an abdominal vein that drains subcutaneous adipose tissue and from a radial artery. Adipose tissue blood flow was measured by xenon washout. Abdominal vein leptin concentrations (mean 8.9 ± 2.4 ng/ml, range 2.1-36.5 ng/ml) were consistently greater than arterial values (mean 6.6 ± 1.9 ng/ml, range 1.7- 28.2 ng/ml) (P < 0.001). The net rate of abdominal adipose tissue leptin production (mean 3.2 ± 0.5 ng · 100 g-1 · min-1) correlated directly with percentage body fat (r(s) = 0.59, P = 0.016). Estimated whole-body leptin production rate (797 ± 283 ng · person-1 · min-1) correlated directly with percent body fat (r(s) = 0.93, P < 0.0001) and with regional leptin production (r(s) = 0.81, P < 0.001). In contrast the rate of leptin clearance from plasma (mean 1.50 ± 0.23 ml · kg-1 · min-1) and plasma leptin half-life (mean 24.9 ± 4.4 min) was unrelated to adiposity (r(s) = 0.06, P = 0.30; r(s) = 0.16, P = 0.30, respectively). These results provide direct evidence that leptin is produced by adipose tissue in humans and that the rate of production is directly related to adiposity. A combination of greater leptin production per unit of body fat and increased production from expanded total body fat mass, rather than alterations in leptin clearance, account for the increase in plasma leptin concentrations observed in obese humans.