Fifteen patients with primary thrombocythemia were seen at the Mayo Clinic from 1960 through 1966. Nine patients presented with hemorrhagic manifestations, especially gastrointestinal bleeding. Three patients presented with thrombotic phenomena, including one patient with the nephrotic syndrome. Platelet counts in excess of 1 million/cu mm were seen in all patients. In this series, 9 patients had hypochromic anemia, and 12 had mature neutrophilic leukocytosis. In three patients there was no splenomegaly in association with Howell-Jolly bodies and target cells in peripheral blood, findings suggestive of infarction atrophy of the spleen. Three patients died within 24 months of diagnosis. Four of the six deaths were related to thrombotic mechanisms. Sodium phosphate P 32 produced more rapid and longer remissions than did busulfan.