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We describe two patients with typical chronic myelogenous leukemia, who at the beginning of the disease lacked the Philadelphia chromosome in bone marrow cells, and 90 and 42 days later, respectively, its presence was shown in all cells analyzed from that tissue. These findings are compatible with the possibility that at least occasionally Ph1occurs secondarily in already leukemic cells. The rapid change from Ph1– to Ph1+ CML in one 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.