An 82-year-old female was diagnosed as having 5q- syndrome accompanied by basophilia, eosinophilia and thrombocytosis. Since cytogenetic abnormalities other than 5q- were also detected, she was considered to be type B of 5q- syndrome. According to the FAB classification a diagnosis of refractory anemia with excess of blasts (RAEB) was made. She was treated with recombinant interferon alpha 2b, because peripheral blood findings resembled those of chronic myelogenous leukemia. Interferon was effective, and resulted in disappearance of peripheral blasts, normalization of platelet numbers, and improvement of basophilia. These changes were interferon-dependent. After 1 year, cytogenetic studies revealed that about 2/3 of metaphases showed normal karyotype. Twenty months after diagnosis, myeloid blastic crisis occurred and eventually the patient died. However, treatment with interferon in this case might support the usefulness of the drugs for this kind of disease.