Adiponectin is an adipocyte-secreted protein that circulates in high concentrations in the serum and acts to increase insulin sensitivity. Previous studies have shown that serum adiponectin is inversely associated with fat mass and insulin resistance in humans and that acute fasting decreases adipose tissue adiponectin mRNA expression in rodents. Whether acute energy deprivation, body fat distribution, or serum hormone levels are associated with circulating adiponectin in humans remains largely unknown. To identify predictors of serum adiponectin levels, we evaluated the association of adiponectin with several anthropometric, metabolic, and hormonal variables in a cross-sectional study of 121 women without a known history of diabetes. We also performed interventional studies to assess whether fasting for 48 h and/or leptin administration regulates serum adiponectin in healthy men and women. Our cross-sectional study shows that, in addition to overall obesity, central fat distribution is an independent negative predictor of serum adiponectin and suggests that adiponectin may represent a link between central obesity and insulin resistance. In addition, estradiol is negatively and independently associated with adiponectin, whereas there is no association between serum adiponectin and leptin, cortisol, or free testosterone levels. Our interventional studies demonstrate that neither fasting for 48 h, resulting in a low leptin state, nor leptin administration at physiological or pharmacological doses alters serum adiponectin levels. Further studies are needed to fully elucidate the physiology of adiponectin in humans and its role in the pathogenesis of insulin-resistant states.