Neuroendocrine response to sepsis may be divided into acute and prolonged phase. As leptin is implicated in the stress response, leptin's profile during both phases, and the possible relationships between leptin and the neuroendocrine response to sepsis were investigated. Thirty adult patients with sepsis in an intensive care unit were studied. Blood samples were collected at the acute and the prolonged phases. In acute sepsis, leptin levels were higher in patients than in controls (10.2 +/- 2.5 vs. 4.1 +/- 1.2 ng/ml, p =0.01) and correlated positively with insulin levels and insulin resistance. A decline in leptin levels was found during prolonged sepsis (from 10.2 +/- 2.5 to 6.2 +/- 1.7 ng/ml, p=0.001), which was not related to survival (p=0.913). At the onset of sepsis, leptin levels increased in correlation with insulin and insulin resistance, possibly indicating a cause-effect relationship. However, the decline in leptin levels during the prolonged phase of sepsis was not related either to survival or to metabolic and hormonal changes.