Alveolar echinococcosis (AE), caused by Echinococcus multilocularis (E.m.), provokes a characteristic immune response. Based mainly on in vitro studies, Th2 dominated immunity is associated with increased susceptibility to disease, while Th1 cell activation is assumed to induce protective immunity. We investigated serum levels of interleukin (IL)4, IL-10, and IL-12 in 40 AE patients and 20 controls to assess Th1/Th2 cell activation in vivo. Significantly higher levels of IL-10 were found in AE patients (P = 0.003) than in controls, with a tendency to higher concentrations in progressive cases. In contrast, IL-4 was only measurable in a minority of patients and controls. IL-12 levels (measured with an ELISA that detects both the p35/p40 heterodimer and free p40) were comparable between AE patients and controls and showed a similar distribution pattern to IL-10 with regard to disease progression. By using an IL-12-ELISA specific for the heterodimer, only minute amounts of IL-12 were detectable in merely a minority of samples. In conclusion, our data are suggestive of Th2 dominated immune response in AE in vivo.