The obesity epidemic has focused attention on the endocrine function of adipose tissue. Adipose tissue secretes leptin, cytokines, complement factors, and components of the coagulation cascade, most of which are increased in obesity. In contrast, a strong negative correlation exists between adiponectin and adiposity, insulin sensitivity, diabetes, vascular inflammation, and atherosclerosis. Adiponectin treatment in rodents increases insulin sensitivity and reduces lipids and atherogenesis. Chronic and central adiponectin treatment reduces weight, glucose, and lipids. The insulin-sensitizing action of thiazolidinediones is mediated, in part, through adiponectin. A causal role of adiponectin in diabetes, dyslipidemia, and atherosclerosis has been established in knockout mice. Therefore, adiponectin seems to be a marker of obesity-related diseases and a potential therapeutic target.