Sixty-five patients with essential thrombocythaemia (ET) on different treatment regimens were studied with regard to EDTA-plasma erythropoietin (EPO) concentrations. In accordance with other studies we found that close to 50% of the untreated ET patients had subnormal (<3.7 IU/L) plasma EPO. The mean plasma EPO concentration for untreated ET patients was significantly lower compared to patients treated with hydroxyurea (HU), radiophosphorous, alpha-interferon or combinations of myelosuppressive agents. This was also true after correction for differences in haemoglobin concentrations had been introduced. An increase in plasma EPO was recorded in all 20 ET patients in whom plasma EPO was registered before and after initiation of myelosuppressive therapy. At the time of diagnosis plasma EPO concentration was available in 31 of the ET patients. In 13 of them the plasma EPO was subnormal whereas the EPO concentrations were > or =3.7 IU/L in the remaining 18 subjects. It was demonstrated that the time to initiation of myelosuppressive treatment was significantly shorter for the former group of patients; they also had more vascular events (11 out of 13) than the group of patients with plasma EPO concentrations > or =3.7 IU/L (9 out of 18). It therefore appears that a subnormal plasma EPO in newly diagnosed ET might be a risk factor for the development of vascular complications.