Thrombotic and hemorrhagic events may result from high circulating concentrations of platelets (> 1,000,000/mm3), and measures to reduce the platelet count are indicated in symptomatic or extreme thrombocytosis. The platelet count can be decreased quickly by plasmapheresis, but the effect is transient. Patients with thrombocytosis secondary to a myeloproliferative disease, such as chronic myelogenous leukemia (CML), frequently require more sustained suppression of the platelet count. Hydroxyurea, busulfan, and interferon are used to maintain a lower platelet count but are occasionally ineffective or intolerable. An alternative to these therapies is anagrelide, a quinazolin derivative that was approved by the Food and Drug Administration in March 1997. Because current dosing guidelines for anagrelide are scarce, the dosing method of the Anagrelide Study Group that published the largest study to date on the drug's efficacy in thrombocytosis was followed. Two unexpected episodes of anagrelide-induced thrombocytopenia occurred despite following these dosing methods. This prompted a critical evaluation of the pharmacodynamic response and the appropriateness of anagrelide dosage recommendations. A case of thrombocytosis treated with anagrelide in a patient with CML is described.