Recent studies have suggested that circulating concentrations of leptin might play a role in cancer cachexia. In the first part of the study, we compared circulating concentrations of free and total leptin, percent fat mass, and the inflammatory markers C-reactive protein (CRP) and interleukin-6 (IL-6), together with appetite score, in age- and gender-matched healthy controls (n = 11) and advanced gastrointestinal cancer patients (n = 26). In the second part of the study, the same measurements were repeated before and after megestrol acetate treatment of weight-losing gastrointestinal cancer patients (n = 10). Body mass index and percent fat mass were significantly lower (P < 0.05) and IL-6 and CRP were significantly higher (P < 0.05) in cancer patients than in controls. There was no difference in the percentage of leptin bound in the circulation between controls and cancer patients. Circulating "free" leptin concentrations correlated with percent fat mass in controls (r = 0.745, P = 0.008) and cancer patients (r = 0.600, P = 0.001). In cancer patients, circulating leptin concentrations, either free or total, were not correlated with IL-6 or CRP concentrations. When adjusted for fat mass, the circulating concentrations of free and total leptin were significantly lower in the cancer patients (P < 0.01). Megestrol acetate treatment significantly increased circulating free and total leptin concentrations in the cancer patients (P < 0.05). There was a significant positive correlation between the change in circulating concentrations of free and total leptin and the change in percent fat mass (r = 0.685, P < 0.05 and r = 0.661, P < 0.05, respectively). The results of the present study indicate that the proportions of free and bound leptin in the circulation do not differ between normal subjects and patients with gastrointestinal cancer and in both groups are related to fat mass. Furthermore, the increase in circulating leptin concentrations after megestrol acetate treatment is not associated with any alteration in leptin binding.