This retrospective study reviewed 94 patients, aged 6 to 90 years, with thrombocythemia. The women/men ratio was 1.76. At onset or, less commonly, during the course of the disease, 45% of the patients presented hemorrhagic manifestations, 29% thrombotic complications, and only 14% the association of hemorrhage and thrombosis. The average platelet count was 1200 X 10(9)/1, with no difference according to sex or age. Platelet hypoaggregation was seen in practically all the cases (94%), although myelofibrosis was less frequent (54%). The frequency of hemorrhages increased when the platelet count was above 1000 X 10(9)/1 (P less than 0.01), but the occurrence of thrombosis was correlated neither with platelet count nor with thrombopathy. Survival time was lengthy: 80% of survival (standard error 6%) was observed at 100 months. Transformation to acute leukemia was observed in five patients. Because the disease course is most often prolonged, therapeutic measures must be conservative: anti-aggregating drugs in small doses, and chemotherapy beginning with nonalkylating agents.