ABSTRACTThe response to iron therapy in 38‐year‐old women was studied in an iron supplementation trial. Women with no or only traces of reticuloendothelial iron in bone marrow samples received in a double‐blind fashion tablets containing 37 mg ferrous iron or placebo three times daily. The women in the iron supplementation group increased significantly more their haemoglobin concentration and packed cell volume than the women in the placebo group. The average increase in haemoglobin concentration was 7.5 g/l. Approximately 75 % of the women in the iron supplementation group increased their haemoglobin concentration in spite of the fact that only 20 % had pretreat‐ment concentrations below 120 g/l. We found no differences in change of haemoglobin concentration, packed cell volume, serum iron concentration, total iron binding capacity or transferrin saturation between subjects with no reticuloendothelial iron and women with only traces of reticuloendothelial iron. We considered it impossible to define “responders” to iron therapy in an acceptable way by means of the iron supplementation test described in this paper.