Abstract: Objective To study the changes of leptin expression in serum of patients with lung, colonic or rectal caner, and to find out the association between leptin expression and prognosis of such patients. Methods Serum samples were collected from patients with lung, colonic or rectal cancer and normal controls. Serum leptin and carcinoembryonic antigen levels were measured with relevant kits, and related biochemical variables reflecting organ metabolic functions were detected with an automatic biochemistry analyzer. Results The leptin expression level was significantly higher in patients with 3 different types of cancer than in normal controls, while different variations were found both in carcinoembryonic antigen level and in organ metabolic functions. Conclusion Leptin may be used as a new variable to evaluate the situation of patients with cancer.