BACKGROUND Ischemic stroke as a presenting sign of essential thrombocythemia has been infrequently reported. We describe six patients in whom cerebrovascular disease was the first manifestation of this myeloproliferative disease. A positive endogenous megakaryocyte and/or erythroid colony growth from blood was a diagnostic criterion of essential thrombocythemia in patients with platelets counts lower than 600 x 10(9)/L. METHODS These six patients represented 0.54% of all patients with first stroke, 42.8% of all hematologic disorders associated with stroke, and 12.5% of all patients with essential thrombocythemia diagnosed from 1986 to 1992 at our institution. Eleven acute cerebrovascular accidents (6 transient ischemic attacks, 5 definitive cerebral infarcts) were registered. Mean time from ischemic stroke to diagnosis of essential thrombocythemia was 4.5 months (range, 1 to 12 months). The mean platelet count was 597 x 10(9)/L (range, 414 to 760 x 10(9)/L). Four patients had platelets counts lower than 600 x 10(9)/L. All patients had circulating erythroid progenitors, megakaryocytic progenitors, or both. CONCLUSIONS Ischemic stroke as a presenting manifestation of essential thrombocythemia is probably underrecognized. The diagnosis of thrombocythemia should not be excluded on the basis of platelet counts lower than 600 x 10(9)/L. The availability of in vitro culture of hematopoietic progenitors from peripheral blood makes it possible to diagnose early and atypical cases.