A female patient with eosinophilia and cardiac symptoms was found to have a unique chromosomal aberration [t(4;7)(q11;p13)] of bone-marrow precursors. The disorder was classified as a chronic myeloproliferative syndrome with eosinophilia. Due to a significant increase in the white blood cell and eosinophil count during initial treatment with prednisone and hydroxyurea, Interferon alpha-2a was administered at a dose of 3-5 x 10(6) I.U. s.c., five times per week, and induced a long-term complete haematological and cytogenetic response. The clinical features of this case are presented and discussed in the context of the current literature.