 This is an 18-week open-label, multicenter study to evaluate the efficacy and tolerability of CEP-701 lestaurtinib treatment at a dosage of 80 mg bid for 18 weeks 126 days in patients with Polycythemia Vera PV who have abnormal baseline neutrophil counts or require hydroxyurea therapy and patients with Essential Thrombocytosis ET who require hydroxyurea therapy for disease control. 