The production of cytokines by helper T lymphocytes is a critical event in the immune response, as alterations in the regulation of this process may result in an appropriate immune response, persistent infection or the development of autoimmune disease. Previously, this group has used flow cytometry to demonstrate the expression of interleukin-12 (IL-12) in peripheral blood CD4+ T lymphocytes from patients and mice with advanced cancer. The aim of the present study was to investigate whether CD4+ T lymphocytes from the peripheral blood (PB) of patients with cancer produce IL-12, using molecular approaches, flow cytometry and cellular imaging techniques. CD3+ and CD4+ cells, and cells producing IL-12, were isolated from the PB obtained from patients with cancer, using a cell sorting flow cytometry technique. The positivity of cells for CD3, CD4 and IL-12, which were identified by cell sorting, was visualized using immunofluorescent cellular imaging. Total RNA was extracted from the CD3+CD4+IL-12+ cells, obtained by cell sorting, for confirmation of the presence of IL-12 mRNA, using reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR demonstrated the presence of IL-12 mRNA in all patients (n=14), in contrast to the control group, in whom IL-12 expression was not detected. Immunofluorescent analysis of CD4+ T lymphocytes showed positive intracytoplasmatic IL-12 staining. These results demonstrated that CD3+CD4+ T lymphocytes in the PB of patients with cancer have the capacity to synthesize and express IL-12.