METHODS Polycythemia vera (PV) and essential thrombocythemia (ET) in young patients are rarely reported. Their natural histories seem to differ from those of older patients and the best treatment is still uncertain. In this follow-up study we have evaluated a cohort of 64 consecutive patients younger than 40 to determine the incidence of thrombohemorrhagic events and the long-term outcome. RESULTS Twenty-eight patients (20 M; 8 F) had PV, and 36 ET (21 F, 15 M). Mean follow-up was 8.2 years (range 4 months-16.7 years) in PV and 6.5 years (range 5 months-15 years) in ET. Thrombohemorrhagic symptoms were present at diagnosis in 10/28 patients (35%) with PV and in 12/36 patients (33%) with ET; during follow-up in 15/28 PV patients (53%) and in 13/36 ET patients (36%). Thrombotic events were the most frequent symptoms, both at diagnosis (52% in PV, 65% in ET) and during follow-up (43% in PV, 52% in ET). A total of 19/28 PV patients (67%) and 17/36 ET patients (47%) had thrombotic complications. Hemorrhagic complications at diagnosis were 4% and during follow-up 13% in PV, and 15% and 13% in ET. A total of 5/28 (18%) PV and 6/36 (17%) ET patients had hemorrhagic events. No laboratory parameter, including platelet count, was predictive of these events. Five PV patients had major thrombotic complications (18%). Four PV patients died (14%), 2 because of ANLL (7%), 2 because of thrombotic events (7%). Four ET patients experienced major complications, in three cases thrombotic (8.3%), in one hemorrhagic. No leukemic transformation occurred in ET and no ET patient died. CONCLUSIONS In our experience, severe thrombohemorrhagic complications are present in young patients with PV and ET, which excludes young age as a favorable prognostic factor. Treatment also seems advisable for young patients and myelosuppressive treatment might be required. Prospective studies are urgently needed to assess the best treatment for this particular subset of patients.