Abstract A patient with chronic granulocytic leukemia is presented with documented central nervous system myeloblastic transformation, while in hematologic remission. This transformation was confirmed by the demonstration of myeloblasts with the Philadelphia (Ph1) chromosome in the cerebrospinal fluid. The course of his illness was unusual in that the initial single manifestation of the acute myeloblastic transformation was an acute meningeal leukemia. Intrathecal Methotrexate treatment resulted in a remission for three months, following which general systemic involvement by acute myeloblastic leukemia ocurred.