Myeloproliferative disorders (MPD) are prone to modification and evolution during the progression of the disease. While post-polycythemia myeloid metaplasia and chronic myelogenous leukemia following polycythemia vera have been frequently described, no report is available about the evolution of polycythemia vera into essential thrombocythemia. Our case is probably the first report on this occurrence. In the course of a fortuitous observation of electrocardiographic alterations, a diagnosis of polycythemia vera was ruled out in accordance with polycythemia vera study group criteria. At the time of diagnosis, RBC was 6 x 10(12)/L, WBC 15 x 10(9)/L, Ht 59% and platelets 1000 x 10(9)/L. The patient was treated with phlebotomies and radioactive phosphorus achieving a good remission or the disease. Five years later, platelets rose to over 3300 x 10(9)/L without significant modification or RBC, WBC and Ht. The restaging or the disease was consistent for an essential thrombocythemia. In particular, RBC mass was within normal levels. During the last ten years, the patient has been followed recurrently and the blood picture remained stationary, without an increase in the hematocrit but with a platelet count between 658 and 800 x 10(9)/L. We conclude that this report may complete data concerning the evolution of MPD in others.