Although it is well established that in polycythaemia vera (PV) both ‘normal’ and ‘abnormal’ erythroid progenitors proliferate, it is less known to what extent the circulating erythrocytes express normal characteristics. We found reduced erythrocyte densities, decreased MCHC, and increased lipid content. These properties, together with increased sialic acid, seem to explain the extremely low sedimentation rate and decreased deformability of polycythaemic blood samples. Other characteristics were high activity of glycolytic enzymes, increased in vitro production of lactate, and a concomitant decline in ATP and 2,3‐diphosphoglycerate. Some of these properties have been described in foetal erythrocytes and are features of normal young red cells. However, they seem to represent true features of PV and not a consequence of younger cell populations. The similarity between mature erythrocytes in PV and in foetal life supports the possibility that the proliferation process in this disease has a mechanism in common with foetal erythropoiesis.