A patient with Ph1 positive chronic myeloid leukemia developed accelerated phase with cytological abnormalities of platelets and megakaryocytes, and persistence of a high platelet count, after the failure of a course of intensive chemotherapy. During this phase, the karyotype analysis revealed two different Ph1 positive clones: one with a pericentric inversion of chromosome 3, the other with the same abnormality and a paracentric inversion of the second chromosome 3. Cases of acute leukemia with normal thrombopoiesis and abnormalities of chromosome 3, especially paracentric inversion, have already been reported. The significance of this association is discussed.