Human adipose tissue has an important protein secretory function. Cytokines, hormones, prohormones and enzymes are secreted from fat cells and act in an endocrine or paracrine fashion. The production of several of these proteins is affected by obesity; normally there is an increase in the obese state. Protein production is, as a metabolic activity, subject to regional variations. In particular, the production of leptin, angiotensinogen, interleukin-6 and plasmin activator inhibitor-1 differs between subcutaneous and visceral adipose tissue sites, but no regional differences have been reported in the production of tumour necrosis factor alpha. It is possible that regional variations in protein production by adipose tissue are of importance in some of the endocrine and metabolic disturbances seen in various forms of obesity, such as visceral and upper-body obesity.