Pathological weight loss is a feature of many diseases and contributes to mortality and morbidity. Although cytokines have been implicated in some models of pathological weight loss, little is known about cellular mechanisms responsible for cachexia in patients with cancer. Leptin is a fat cell product that acts centrally to reduce appetite and decrease metabolism. Leptin synthesis is stimulated by cytokines, and circulating levels of cytokines are elevated in some cancer patients. We hypothesized that cytokine-induced hyperleptinemia contributes to pathological weight loss in patients with pancreatic cancer. To evaluate this hypothesis, fasting serum leptin concentrations were measured in 64 patients undergoing surgery for pancreatic cancer. Preoperative interviews were used to assess body weight and appetite history. Thirty of 64 pancreatic cancer patients had cachexia (weight loss of >10% over the 6 months before surgery). Self-reported loss of appetite was associated with the presence of cachexia. Leptin concentrations, when corrected for body mass index, were lower than levels reported in healthy humans. Six patients had leptin levels more than 2 times those predicted by body mass index. There was no association between patients with increased leptin concentration and weight loss or anorexia. We conclude that a reduced appetite contributes to weight loss in patients with pancreatic cancer. High plasma leptin levels, however, do not appear to contribute to cachexia in these patients.