1. Adipocyte-derived leptin is postulated to represent the afferent hormonal signal to the hypothalamus in a feedback mechanism that regulates fat mass. In this proposed feedback mechanism, increased fat mass leads to an elevated plasma leptin level that eventually induces a decrease in appetite and an increase in energy expenditure, and vice versa. 2. As anorexia and hypermetabolism play a role in the development of cancer cachexia, we investigated the hypothesis that underlying abnormalities in the leptin feedback mechanism (in particular relatively high plasma leptin levels or, on the other hand, a hypothalamic insensitivity to a fall in leptin levels) might be involved. For this purpose, total plasma leptin, body composition (fat mass and fat-free mass), appetite and resting energy expenditure were assessed in 21 male lung-cancer patients. 3. Total leptin was detectable in six patients and non-detectable in 15. In comparison with the latter, the patients with detectable leptin were characterized by a trend towards less weight loss (3.4% compared with 11.0%, P = 0.07), as being less underweight (body mass index 23.8 kg/m2 compared with 19.4 kg/m2, P = 0.004) and by a higher fat mass (21.4 kg compared with 9.7 kg, P = 0.001). Significant between-group differences in appetite and resting energy expenditure were lacking. 4. Based on these findings, we conclude that in cancer the afferent part of the leptin feedback mechanism functions normally and that, in particular, elevated leptin levels are not involved in the development of cachexia. Since the absence of plasma leptin was not associated with an increased appetite and decreased energy expenditure, disturbances in the hypothalamic part of the feedback mechanism are hypothesized.