A 57-year-old woman visited to our hospital complaining of paresthesia in the right leg. She had no abnormal physical findings. However, the peripheral blood examination demonstrated 7% basophilia with 8000/microliters WBC count and decreased neutrophil alkaline phosphatase activity (score 37, rate 19%). She was diagnosed as Ph1 chromosome positive CML in early phase by the chromosomal analysis of bone marrow cells. She received subcutaneous injection of natural interferon-alpha at a dosage of 600 x 10(4) IU daily from March 10, 1987. The dosage and administration interval were gradually reduced and prolonged. Since November 1988, weekly injections of 300 x 10(4) IU has been administered as maintenance therapy. Cytogenetic improvement was seen at 4 months after the start of IFN. Disappearance of Ph1 chromosome positive cells was observed on December 11, 1987. It was suggested that the administration of IFN from the early chronic phase played an important role in the control of the disease.