Eosinophilic cellulitis (Wells' syndrome) is a rare disorder characterized clinically by recurrent erythematous plaques resembling cellulitis and histologically by a dermal infiltrate of lymphocytes, eosinophils and eosinophil debris between collagen bundles, forming flame figures in typical cases. A 71-year-old woman with Wells' syndrome with blood and bone marrow eosinophilia showed a good response to dapsone. The level of eosinophil cationic protein (ECP) in serum was elevated. Immunophenotyping of peripheral T cells revealed an increased proportion of CD3+CD4+T cells. The patients' cultured peripheral lymphocytes spontaneously released significant amounts of interleukin 5 (IL-5), but not interleukin 4 (IL-4) or interferon gamma (IFN gamma). These findings suggest that activated T cells may be involved in the pathogenesis of blood and tissue eosinophilia in this patient.