Interleukin-2 (IL-2) and interleukin-12 (IL-12) may represent the most important antitumour cytokines in human neoplasms. IL-2 blood levels decrease in advanced solid malignancies, but currently there are no data on IL-12 secretion in cancer patients. This study was performed to obtain preliminary data about IL-12 secretion in patients with solid malignant tumours, either in relation to the extension of disease, or to other cytokines, including IL-2, IL-6 and IL-10. The study included 40 solid cancer patients, 24 of whom showed distant organ metastases. Cytokine serum levels were measured by an enzyme immunoassay of blood samples collected during the morning. No patient had abnormally low levels of IL-12, but the levels were high in 14/14 (35%) patients. Mean levels of IL-12 were significantly higher in metastatic patients compared with non-metastatic patients (P < 0.05). Moreover, metastatic patients with high blood concentrations of IL-12 showed significantly lower levels of IL-10 than metastatic patients with normal IL-12 values, while no difference was seen in IL-2 mean concentrations. IL-6 mean levels were lower in metastatic patients with increased IL-12 levels, but this was non-significant. This preliminary study shows that advanced solid cancers are not characterised by a diminished secretion of IL-12, but rather IL-12 levels tend to be abnormally high in metastatic cancer patients.