OBJECTIVE To evaluate the immunological status of patients with gastric cancer before surgery, we investigated the relationship between serum interleukin-12 (IL-12) levels and clinicopathological factors. METHODS We measured serum IL-12 levels in 127 patients with gastric cancer and 35 healthy controls, by a sandwich enzyme-linked immunosorbent assay using the Human IL-12 +p40 Immunoassay kit. RESULTS The serum IL-12 levels in the patients with gastric cancer were significantly higher than those of the healthy controls (P < 0.05). There were no significant differences in disease stage or gross appearance among the cancer groups, but the serum IL-12 levels in patients with T4 disease were significantly lower than those in patients with T1, T2, or T3 (P < 0.01). There were no significant differences in serum IL-12 levels between patients with and those without lymph node, liver, or peritoneal metastasis. The serum IL-12 levels in patients with distant metastasis were significantly lower than those in patients without distant metastasis (P < 0.02). There were no significant differences in the serum IL-12 levels according to classification by histopathological findings. Analysis with the linear correlation coefficient showed no significant correlation between serum IL-12 and serum carcinoembryonic antigen, carbohydrate antigen (CA) 19-9, CA 72-4, alpha-fetoprotein, or immunosuppressive acidic protein. However, there was a significant relationship between serum IL-12 levels and soluble IL-2 receptor levels (r = 0.53, P < 0.01). CONCLUSIONS Serum IL-12 levels in patients with far-advanced gastric cancer were significantly lower than those in patients with less-advanced gastric cancer. This is because macrophages in patients with far-advanced cancer would be hectic and unable to produce sufficient IL-12.