Wells' syndrome, or eosinophilic cellulitis, is a rare dermatosis characterized histologically by a dermal infiltrate of eosinophils, lymphocytes and histiocytes between collagen bundles and amorphous or granular eosinophilic deposits on collagen, constituting flame figures. We report a 54-year-old woman with eosinophilic cellulitis whose peripheral blood showed a marked eosinophilia and a high proportion of CD4+CD7− cells before treatment. Reverse transcriptase—polymerase chain reaction revealed that CD4+CD7− cells, but neither CD4+CD7+ nor CD4−CD8+ cells, in the circulating mononuclear cells expressed mRNA for interleukin (IL)-5, the major cytokine involved in eosinophilia. The proportion of CD4+CD7− cells decreased, and expression of mRNA for IL-5 disappeared in the peripheral blood, when the disease was treated by the administration of intravenous recombinant interferon-γ. These findings suggest that circulating CD4+CD7− T cells play a pivotal role in the pathogenesis of eosinophilic cellulitis by producing IL-5.