A clinical phase (spent phase) in the course of polycythaemia vera (PV) cases is described as enlargement of the spleen in spite of treatment, frequent cytopenia of one or several lines, persistent red cell hypervolaemia with considerable increase of plasma volume, persistence of myeloid hyperplasia with no collagen myelofibrosis or osteomyelosclerosis, absence of hepatosplenic erythroblastic metaplasia, as shown by radio-iron kinetics and/or 111In-transferrin scintigraphy. The frequency of this phase was 5% in a study where it was not systematically sought, but it could in fact be greater. Its occurrence is not related to the clinical and biological parameters of PV. On the other hand, it is significantly more frequent and earlier in patients treated by phlebotomies than in those treated by myelosuppression (32P). In four of the 12 cases, this phase was rapidly followed by an acute leukaemia. In eight cases there was a 1-5 year interval before a myelofibrosis with splenic myeloid metaplasia. This evolution could at this stage be delayed by chemotherapy. The efficacy of splenectomy should be studied.