Sixty-three bone marrow (BM) and peripheral blood specimens from patients with platelet counts of 1000 x 10(9)/L or greater were examined in an attempt to determine if any BM or peripheral blood findings could be used reliably to distinguish primary thrombocythemia from other myeloproliferative disorders and extreme examples of reactive thrombocytosis. Our results indicated that the BM findings in primary thrombocythemia were quite similar to those in polycythemia vera and chronic granulocytic leukemia with associated extreme thrombocytosis. However, statistically significant differences between the BM findings in myeloproliferative disorders and extreme reactive thrombocytosis were found in the numbers of megakaryocytes, presence or absence of megakaryocyte clusters, stainable iron, cellularity, and reticulin content. We concluded that BM examination is a useful procedure as an aid in determining the cause of extreme thrombocytosis.