A clinicopathological study was performed on 46 patients with chronic myeloproliferative diseases (CMPD) showing a thrombocythemia in excess of 1,000 x 10 (9)/liter. When applying rigid diagnostic criteria only 23 patients were compatible with the initially suspected diagnosis of primary thrombocythemia (PTH). Comparison of PTH with the other entities of CMPD (CGL, 10, AMM, 6, and polycythemia, 7 cases) revealed a sustained elevation of the platelet count observable over a period of 2 to 8 years, no marked leukocytosis or abnormalities of the differential blood count, and a normal score of the leukocyte alkaline phosphatase. Episodes of hemorrhage and thrombosis as well as neurological symptoms (paresthesias, dizziness, headache), were encountered frequently as clinical manifestations in PTH. Survival time in PTH was significantly longer than in CGL with accompanying thrombocythemia. In a consecutively biopsied population of patients with CMPD, incidence of PTH was about 8%. In PTH the characteristic histopathology of the bone marrow consisted of an isolated (monolinear) proliferation of the megakaryocytes (density 127+/-47/mm2) without gross abnormalities of this cell lineage or a conspicuous increase in neutrophilic granulo-or erythrocytopoiesis. These lesions are significantly different from the morphological findings in the other CMPD with extreme thrombocytosis.