To investigate the involvement of immunological variables, including the Th1/Th2 balance, in the mechanisms underlying seronegativity in cystic echinococcosis (CE), Th1 and Th2 cytokines were evaluated in ELISA determining IFN-gamma, IL-4, -5 and -10 production in peripheral blood mononuclear cell (PBMC) cultures from 16 patients (six seronegative and ten seropositive) with CE and from six healthy donors. Patients who had undetectable antibody levels in their sera invariably produced no IL-5 and scarce IL-4 and -10 (Th2 response); both groups of patients produced similar amounts of IFN-gamma; (Th1 response). The analysis of host clinical features and cyst characteristics shows that these variables do not explain the lack of detectable serum antibody levels. These findings suggest that seronegativity arises because unknown host or parasite factors, or both, cause inadequate Th2 cell activation thus reducing production of Th2 cytokines, including IL-5, the cytokine primarily implicated in immunoglobulin expression in CE.