The influence of pharmacological treatment on the immune response of patients with Echinococcus granulosus infection was evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR) to determine mRNA expression for IL-12 p35, IL-12 p40, interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha) and IL-4 in PBMC from 12 patients before and after chemotherapy and from seven uninfected controls. Most patients' PBMC showed measurable amounts of IL-12 p35, IL-4, IFN-gamma and TNF-alpha mRNA in parasite antigen-stimulated and unstimulated cultures. Conversely, IL-12 p40 mRNA was detected almost exclusively in successfully treated patients (86%) after therapy. In these patients semiquantitative analysis of RT-PCR products showed a significant difference between IL-12 p40 mRNA mean levels before and after therapy (P = 0.03 in parasite antigen-stimulated cultures; P = 0.001 in unstimulated cultures). IL-4 mRNA was weakly expressed before therapy and more highly so after treatment in both groups of patients and under both culture conditions; IL-4 mRNA reached its highest level in post-therapy PBMC from patients in whom therapy failed (stimulated cultures). IFN-gamma and TNF-alpha mRNA expression increased in patients who responded to therapy and decreased in patients who did not. In contrast to IL-12 p35, IFN-gamma and TNF-alpha mRNAs, IL-12 p40 and IL-4 mRNAs were detected exclusively in patients, suggesting a close relationship between these two cytokines and cystic echinococcosis. Our findings indicate that chemotherapy influences the immune response, thus determining changes in Th1/Th2 cytokine mRNA patterns, predominantly in IL-12 p40 and IL-4 mRNA expression.