Two patients with typical chronic myelogenous leukemia [CML] are described. At the beginning of the disease, these patients lacked the Philadelphia chromosome in bone marrow cells. At 90 and 42 days later, the presence of the Philadelphia chromosome was shown in all bone marrow cells. At least occasionally, Ph1 occurs secondarily in already leukemic cells. The rapid change from Ph1- to Ph1+ CML in 1 of the patients (42 days) suggests the possibility that in addition to Ph1+ cells enjoying marked selective advantage, this change is induced simultaneously in multiple bone marrow cells.