IL-2 and IL-12 are the main antitumor cytokines in humans. Endogenous IL-2 production is generally low in metastatic cancer patients. In contrast, preliminary data suggest a possible increased secretion of IL-12 in a subgroup of metastatic cancer patients, of which the prognostic significance is still to be established. This preliminary study was performed to investigate the survival time in relation to IL-12 blood levels in patients with untreatable metastatic solid tumors. The study included 40 patients (lung cancer: 16; gastrointestinal tumors: 24). Abnormally elevated serum levels of IL-12 were observed in 15/40 (37%) patients, without any apparent relation with tumor histotype. The 1-year survival rate was significantly higher in patients with elevated IL-12 baseline values than in those with normal concentrations (8/15 vs 3/25, p < 0.01). The results of this preliminary study, which need to be confirmed in a greater number of patients by monitoring the clinical course of the neoplastic disease, seem to suggest that abnormally high baseline serum levels of the antitumor cytokine IL-12 may have a favorable prognostic significance, as they appear to be associated with a longer survival time.