The course and complications of idiopathic erythrocytosis.

The presenting features, haematological and clinical course of 30 patients with unexplained high venous haematocrit have been analysed. At presentation these patients had definite polycythaemia, but lacked other features which would permit their classification as primary proliferative polycythaemia (PPP). In the follow-up period, twelve patients (40%) developed features within 6 years of diagnosis which allowed them to be reclassified as PPP. It is suggested that, because of this transition, the term idiopathic erythrocytosis is a more suitable initial description of this group than, for example, benign erythrocytosis. The high incidence of vascular complications at presentation, and the fact that a cerebrovascular accident was responsible for the death of five out of the seven patients who have so far died, is discussed and compared with similar events in PPP. The existence of a pure line red cell proliferation cannot be confirmed or excluded by the observations in the present group of patients. The findings do indicate that follow-up and observation of such patients may establish causes for polycythaemia not in evidence or considered at presentation.