Over 20 years, 58 cases of PV in young people (46 meeting the full PVSG criteria, 12 with elevated red cell volume and leucocytosis or thrombocytosis, without splenomegaly) were studied and have been followed for periods of 3-24 years. These cases represent approximately 5% of the cases of PV referred to the Department of Nuclear Medicine of St Louis Hospital during this period. They differ from older patients in the initial clinical severity, the short interval between the first symptoms and the diagnosis, frequent presentation with a life-threatening complication (two cases of hepatic vein thrombosis, six thrombotic or haemorrhagic events, six splenectomies, two abortions) and a very enlarged spleen in half the cases. However, after the initial complications, the overall survival is very long (exceeding 70%, even when including the initial complications, at 15 years). The vascular accidents occur exclusively in the phlebotomized patients, the main risk factor being the poor stability of the haematocrit. Only one acute leukaemia was observed among the 14 cases treated by radioactive phosphorus and/or alkylating chemotherapy. The most frequent late complication was evolution towards myelofibrosis. This spent phase seemed to occur earlier in patients treated by phlebotomy. On the basis of this data, we would advise the following therapeutic strategy: phlebotomies, as soon as the diagnosis is established, and a systematic long-term treatment by hydroxyurea with the hope of reducing the number of vascular complications and of delaying the evolution towards the spent phase and the myelofibrosis.