Interleukin 12(IL-12) is a heterodimeric cytokine that has potent anti-tumor and anti-metastatic activities. Although clinical trials of recombinant human IL-12 have begun in patients with several advanced malignancies, very few studies have investigated the preoperative serum levels of IL-12 in patients with gastric or colorectal cancer. The purpose of the present study was to investigate the relationships between the preoperative serum levels of IL-12 and clinicopathological factors in patients with gastric or colorectal cancer. Blood was obtained before surgery from 14 patients with gastric cancer and 15 patients with colorectal cancer. Serum levels of IL-12 was assessed using the quantitative sandwich enzyme immunoassay technique. Although not statistically significantly, low serum levels of IL-12 tended to be associated with gastric cancer patients who were node-positive, CEA positive, had tumors that penetrated the serosa, had tumors greater than 5 cm in diameter, were more than 60 years-old, or were more advanced than stage IIIA(TNM) or stage IIIa(Japanese Research Society for Gastric Cancer). Patients with colorectal cancer who were node-positive, had tumors that penetrated the serosa, were more than 60 years-old, or were more advanced than stage III(TNM), stage IIIa(Japanese Society for Cancer of the Colon and Rectum) and Dukes' C also tended to have low serum IL-12 levels. These results suggest that low serum levels of IL-12 may be observed in more advanced gastric and colorectal cancer patients. Thus, patients with low serum levels of IL-12 in gastric or colorectal cancer may require additional immunochemotherapy after surgery.