Eosinophilic fasciitis (Shulman's syndrome) and eosinophilic cellulitis are part of a spectrum of diseases characterized by tissue and peripheral blood eosinophilia. Eosinophils are implicated directly in the lesional process that characterizes these conditions, because signs of eosinophil activation and degranulation are observed at the sites of tissue injury. The cause and pathogenesis of eosinophilic fasciitis and cellulitis are presently unclear. Herein, we report a patient manifesting rapidly progressive localized cutaneous induration of the arms and legs with eosinophilia, no signs of systemic sclerosis, and histopathologic features compatible with the diagnosis of eosinophilic fasciitis. Four years after the onset of eosinophilic fasciitis, the patient had recurrent episodes of eosinophilic cellulitis. Blood screening for clonal T-cell receptor gamma gene rearrangements revealed several amplified clonal populations of circulating T cells. Furthermore, in vitro analysis of cytokine production by the patient's peripheral blood mononuclear cells demonstrated strongly increased production of interleukin 5, the synthesis of which could be completely blocked by interferon (IFN)-alpha. The coexistence of eosinophilic fasciitis and cellulitis in a patient with an abnormal circulating T-cell clone and increased IL-5 production are unique and might be responsible for the eosinophilia and eosinophil-mediated tissue injury. Although not assessed in vivo in this patient, our in vitro data provide a rationale for the use of IFN-alpha in eosinophilic fasciitis and/or cellulitis.