Immunoreactive erythropoietin was estimated in the sera of 23 Nigerian children with protein-energy malnutrition (PEM) and 14 healthy Nigerian children of similar age attending a well baby clinic. The geometric mean estimate for this parameter was 262 mIU/ml (observed range 39-1340 mIU/ml; 95% confidence range 25-1738 mIU/ml) in the children with PEM and 80 mIU/ml (observed range 43-257 mIU/ml; 95% confidence range 27-241 mIU/ml) in the health children. Erythropoietin levels were above the 95% confidence range for the healthy children in 14 of the cases of PEM. There was a statistically significant inverse correlation between the haemoglobin levels of the children with PEM and the logarithm of immunoreactive serum erythropoietin estimates (r = -0.73; P less than 0.001). By contrast, statistically significant correlations were not found between the logarithm of erythropoietin estimates and either the percentage of erythroblasts in the marrow, the M/E ratio or the logarithm of the absolute blood reticulocyte count. These data suggest that there is no abnormality of erythropoietin production in PEM and that the anaemia seen in this condition results from an impairment of erythropoiesis. A stepwise multiple regression analysis revealed a positive correlation between the logarithm of the erythropoietin level and the logarithm of the concentration of circulating neutrophil metamyelocytes plus myelocytes and we speculate on the aetiology of this finding.