An aggressive treatment program combining splenic irradiation, splenectomy and intensive cytotoxic combination chemotherapy was used in 37 patients with Ph1 chromosome positive chronic myelogenous leukemia. Cytogenetic Ph1 chromosome determination on marrow cells revealed a significant but transient decrease in the Ph1 positive cell population in 12/37 (32%) cases. Survival duration appears longer in patients who show a reduction in the Ph1 positive population. Blastic transformation occurred in 18/37 (49%) cases, not different from our historical control.