Human endogenous retroviruses (HERV) and herpesviruses are increasingly associated with the pathogenesis of the neurological inflammatory disease multiple sclerosis (MS). Herpesviruses are capable of HERV activation and simultaneous presence of HERV and herpesvirus antigens have a synergistic effect on cell-mediated immune responses, which tend to be higher in MS patients in comparison with healthy individuals. Here, we investigate whether these synergistic immune responses are reflected in changes in the production of proinflammatory cytokines. Using enzyme-linked immunosorbent assays (ELISAs), we have performed a comparative study between MS patients and healthy controls to investigate the production of interferon (IFN)-gamma, interleukin (IL) 2, or IL-10 as well as the balance between Th1 and Th2 responses in supernatants from peripheral blood mononuclear cells (PBMC) stimulated with HERV and herpes antigen combinations. We have found a significant disproportion in Th1/Th2 responses in PBMCs from MS patients caused by the joint presence of HERV and herpes antigens. The results also showed a significantly higher IFN-gamma production in cells from MS patients; additionally, this production correlated with the synergistic cell proliferations whereas we did not find such a correlation in healthy controls. Our findings suggest that the increased production of IFN-gamma and the induced imbalance in Th1/Th2 responses favouring the inflammatory reactions in MS patients may lead to progression of the disease.