We report a 61-year-old man with essential thrombocythemia (ET) whose clinical course was followed for 12 years. The ET evolved into true idiopathic myelofibrosis (IM) 6 years after the initial diagnosis and progressed to myeloid blastic transformation 6 years later. The cytogenetic analysis showed a normal karyotype during the ET phase but subsequent analysis revealed an abnormal karyotype during the IM phase which evolved clonally at blastic crisis with constant involvement of chromosome 13q and chromosome 7. The close monitoring of essential events, using clinical, morphologic, immunologic and cytogenetic parameters, allowed us to carefully identify the transition from one chronic myeloproliferative disease (MPD) to another. This is only the second case reported showing a clinical evolution of this nature. The clinical and biological aspects of the disease are briefly discussed.