Leptin is an anorexia inductor peptide produced by adipocytes and related to fat mass. Leptin is also produced by fat under proinflammatory cytokine action. Our objective is to study serum leptin levels in relation to nutritional status and acute phase response in advanced-stage non-small cell lung cancer. Seventy-six patients newly diagnosed of non surgical non-small cell lung cancer before chemotherapy treatment and 30 healthy controls were included. BMI, serum leptin and cholesterol levels and lymphocyte count were decreased in lung cancer patients. Cytokine IL-6, TNF-α, sTNF-RII, sIL-2R, IL-12, IL-10 and IFN-γ, and other acute phase reactants as α1 antitrypsin, ferritin, CRP and platelets were all raised in patients, whereas the IL-2 was decreased. We found a direct relationship between leptin and other indicators of the status of nutrition, especially total fat mass. We also found a close relationship between the status of nutrition and the performance status (Karnofsky index). However, serum leptin and nutritional status were inversely correlated with acute phase proteins and proinflammatory cytokines, suggesting a stress-type malnutrition. Although serum leptin levels, nutritional status and Karnofsky index are related to survival, at multivariate analysis they all were displaced by the acute phase reaction markers. These results suggest that cancer anorexia and cachexia are not due to a dysregulation of leptin production. Circulating leptin concentrations are not elevated in weight-losing cancer patients and are inversely related to the intensity of the inflammatory response. In advanced lung cancer patients serum leptin concentrations only depend on the total amount of fat.