A patient's clinical course with Phi‐positive chronic myelogenous leukaemia is described. Blastic transformation was associated with a double Ph1 chromosome and megaloblastosis. During remission, the megaloblastosis disappeared and the cytogenetic pattern reverted to a single Ph1 chromosome. A second blast crisis occurred terminally with a reappearance of aberrant red cell maturation and a double Ph1 chromosome and aneuploidy. The possible significance of these observations to the clone theory of blastic crisis evolution is discussed.