Churg-Strauss syndrome (CSS) is characterized by a granulomatous vasculitis of multiple organs with the cardiovascular system being commonly affected. The initiation of the disease is not well understood, but immunologic phenomena are thought to contribute at least partially to the syndrome. We have studied endomyocardial biopsy specimens from a patient with CSS and eosinophilic myocarditis for characterization of infiltrating immune cells by immunofluorescence staining techniques and found a major population to be composed of CD83+CD14-CD19-CD56-HLA-DR+ dendritic cells (DCs). Further phenotypic characterization of infiltrating CD83+ cells in CSS myocarditis showed a surface profile reminiscent of immature DCs. In the same patient, the cytokine expression pattern of mitogen-stimulated peripheral blood mononuclear cells revealed a TH0 response as evidenced by multiplex polymerase chain reaction, and interleukin-5 levels were elevated in plasma analyzed by enzyme-linked immunosorbent assay. Thus, in this case of CSS myocarditis, we found DC-like cells in myocardial lesions, which may suggest that DCs are involved in the inflammatory process possibly triggered or sustained by an imbalance of DCs and their failure to confer tolerance to self-antigens.