In five patients with vasculitis, hypereosinophilia, and elevated serum IgE levels a diagnosis of Churg-Strauss syndrome was established. To identify a possible role of IgE in pathogenic mechanisms leading to the vasculitis, we performed a sequential precipitation of the patients' sera with different concentrations of polyethylene glycol (PEG) 6000. Using a radio immunosorbent test, we tested the precipitates obtained for IgE. Considerable amounts of IgE were traced in the serum precipitates of all patients, especially after the second precipitation step (4.0% PEG). In contrast, no IgE-containing precipitates were detectable in sera from patients with different allergic diseases and high IgE serum levels. Together with an increase in C3d serum levels and the failure to demonstrate C1q-binding material in patients' sera, these data suggest the involvement of IgE-containing immune complexes in the pathogenesis of Churg-Strauss vasculitis, activating the complement via the alternate pathway.