On comparing lymphocyte patterns in patients with polycythaemia vera (PV) versus healthy blood donors (NC) and patients with secondary polycythaemia (SP), we observed an altered CD4/CD8 ratio (p < 0.01) mainly due to the decreased CD8 subpopulation in PV. We found an increased lymphocyte activity in PV patients when compared with both SP and NC: the interleukin 2 (IL-2) production was significantly higher (p < 0.001) as was the lymphoproliferative response both to phytohaemagglutinin (p < 0.001) and IL-2 (p < 0.005). No significant differences were found between NC and SP. None of the PHA-stimulated PBMC samples from PV, SP or NC produced erythropoietin (EPO)-like or EPO-inhibitory activity. Our findings suggest an altered lymphoid lineage in PV patients.