Anagrelide is an oral imidazoquinazoline agent with an anti-cyclic AMP phosphodiesterase activity and inhibits platelet aggregation in both humans and animals. In addition, it has in humans a species-specific platelet-lowering activity observed at dose levels lower than those required to inhibit platelet aggregation. Because of this, the drug has been tested in patients with clonal thrombocytosis and has been shown to have potent platelet-reducing activity in essential thrombocythemia (ET) and related disorders. The mechanism of action may involve the drug's interference with megakaryocyte maturation. More than 90% of patients with ET respond to anagrelide regardless of the presence or absence of previous therapy. The responses are durable with a median maintenance dose of approximately 2 to 2.5 mg/day. Side effects are related mostly to the drug's direct vasodilating and positive inotropic effects and include headache, fluid retention, tachycardia, and arryhthmias. The place of anagrelide therapy in the current management of patients with ET is discussed.