The 61 observations of primary thrombocythemia described in this report represent approximately 15% of the cases of polycythemia vera recorded by the authors over the same 18-year period. The group includes 35 females and 26 males, with a mean age of 62. The disease is usually discovered on routine blood tests (half of cases), and more rarely because of hemorrhagic or thrombotic manifestations. Splenomegaly is found in one-third of cases. Platelet count is permanently above 800 X 10(9)/l (mean : 1 500 X 10(9)/l); mild hyperleukocytosis (mean : 16 X 10(9)/l) with predominant neutrophil polynuclears is usual but myelemia is not constant (28% of cases) and always very moderate; red cell parameters are normal in three-fourths of cases, while the remaining patients have anemia, either due to iron depletion or not. Reticulinic myelofibrosis, usually minimal, is found in 40% of cases. Medullary karyotype is always normal, without chromosome Ph1. Platelet functional abnormalities are not constant and do not correlate with the magnitude of thrombocythemia. 51 patients (84%) received myelosuppressive therapy, mainly by busulfan or radioactive phosphorus. Most deaths were due to intercurrent causes and only one patient developed acute leukemia. 71% of patients are alive at five years and subsequent decrease in the actuarial survival curve is very gradual.