Animal research suggests that leptin may have an important role in the regulation of energy balance. The role of leptin in the progressive involuntary weight loss associated with cancer in humans is of considerable interest. However, such studies are limited. In this study, we compared circulating leptin concentrations in gastrointestinal cancer patients and weight loss (n = 27) with those of healthy subjects (n = 27). The effect of the presence of an inflammatory response on leptin concentrations was also examined. There were significantly lower leptin concentrations in male (median, 2.4 microg/liter; range, <0.5-6.0 microg/liter) and female (median, 3.4 microg/liter; range, <0.5-9.8 microg/liter) cancer patients than there were in male (median, 6.5 microg/liter; range, 3.1-10.9 microg/liter) and female (median, 18.7 microg/liter; range, 8.0-31.5 mcirog/liter) healthy subjects (P < 0.001). However, the leptin concentrations in both patients and normal subjects were related to the predicted percentage of body fat (r = 0.731; P < 0.001). Circulating leptin concentrations in the cancer patients were not altered by the presence of an inflammatory response. These results suggest that cancer anorexia/cachexia is not due to a simple dysregulation of leptin production.