A 43-year-old woman with a 10-year history of bronchial asthma developed marked peripheral blood eosinophilia and symptoms of vasculitis. A diagnosis of Churg-Strauss syndrome (CSS) was made, and her symptoms were successfully treated with low-dose prednisolone. Serum rheumatoid factors (RF) of both IgG and IgM, but not IgA or IgE, subclasses transiently appeared in accordance with the vasculitic phase. Serum interleukin-6 (IL-6) levels also transiently increased in this phase and the peak level was reached just prior to the maximum of RF elevation, suggesting the role of IL-6 as an inducer of RF. RF and IL-6 seemed to be involved in the pathogenesis of the vasculitis in this patient.