Patients with malignant disease frequently develop anaemia. To investigate the role of erythropoietin (EPO) in this anaemia, serum levels were determined in patients with solid tumours. The study population consisted of 84 patients (44 males, 40 females) with solid tumours and 99 healthy control subjects, and 13/84 patients were anaemic. Serum EPO was clearly elevated in the anaemic tumour patients, but this increase was less than in patients suffering from iron deficiency anaemia. As in iron deficiency anaemia, the correlation between EPO levels and haemoglobin values was inverse. When compared to healthy control subjects, the levels of EPO in the tumour patients without anaemia were decreased. We conclude that there may be an inhibition in the expression or secretion of EPO in patients with solid tumours which, as yet, has not been further defined. Based on this, the treatment of anaemia in cancer patients with erythropoietin appears promising.