A 41-year-old man with severe hypereosinophilic syndrome was first given a high dose of steroids. His condition improved, and the dose of steroids was tapered. His condition then worsened, and administration of cyclosporin was begun. Disease activity decreased and eosinophil counts decreased to the normal range. Cyclosporin and steroids inhibit the interleukin-2 gene transcription factors NF-AT and AP-1. They also inhibit the interleukin-5 production by peripheral lymphocytes stimulated by interleukin-2. AP-1 is the primary target for steroid-mediated repression of IL-2 gene transcription, but NF-AT appears to be the main target of cyclosporin. In patients with the hypereosinophilic syndrome, cyclosporin can be effective and can minimize the adverse effects of long-lasting therapy with high doses of steroids.