Abstract Measurements of erythropoietin (Ep) levels in patients with the anaemia of chronic disorders due to malignant disease have given variable results. This variation may be due to the wide range of malignancies studied and the assay method (whole animal) used. In this study Ep levels were measured, using the foetal mouse liver assay, in 39 patients with lung cancer and 19 controls. Twelve patients had reduced haemoglobin levels (Hb less than 11.5 g/dl for males and 11 g/dl for females) and the features of the anaemia of chronic disorders. Their mean Ep level was 0.21 iu/ml. This was significantly lower than for the normal controls, whose mean value was 0.31 iu/ml (P < 0.02). This data supports the concept that lack of an appropriate Ep response to anaemia is one factor in the genesis of anaemia in malignancy.