Anagrelide is an imidazokinazoline derivate that reduces platelet production by interfering with megakaryocyte proliferation and maturation. As a non-cytostatic drug it selectively affects megakaryocyte lineage and therefore anemia and leukocytopenia are not likely to occur. This makes anagrelide adequate for the treatment of chronic myeloproliferative disorders characterized by marked thrombocytemia. In this study we have evaluated efficacy of anagrelide in 14 pretreated patients with essential thrombocytemia. The response was achieved in 11 patients (78%) and was defined as a platelet count lower than 450 x 10(9)/l or 700 x 10(9)/l without thrombohemorrhagic incidents. The therapy was stopped in 6 patients; three patients did not respond to treatment; one had a serious side effect; pregnancy was the reason for discontinuation of therapy in one patient, and in one patient therapy was changed by his own request. We can conclude that anagrelide is an effective and safe drug for pretreated patients with essential thrombocythemia.