Chronic phase chronic myelogenous leukemia is a phase of chronic myelogenous leukemia in which 5% or fewer of the cells in the blood and bone marrow are blast cells (immature blood cells). This phase may last from several months to several years, and there may be no symptoms of leukemia. Chronic phase chronic myelogenous leukemia is a phase of chronic myelogenous leukemia in which 5% or fewer of the cells in the blood and bone marrow are blast cells (immature blood cells). This phase may last from several months to several years, and there may be no symptoms of leukemia. This article incorporates public domain material from the U.S. National Cancer Institute document 'Dictionary of Cancer Terms'.