We used magnetic resonance imaging (MRI) to study vertebral bone marrow in hemodialysis patients during treatment with recombinant human erythropoietin (rHuEPO). We found changes in T1 relaxation times and image contrast within 14 days after starting treatment, before any response was seen in the hemoglobin concentration in peripheral blood. The increase in T1 relaxation times, together with earlier reported changes observed with localized magnetic resonance spectroscopy, indicate an alteration in cellular composition of the hemopoietic bone marrow with an increase in the amount of hemopoietic active tissue. MRI may be a useful, non-invasive way of evaluating bone marrow response to different modes of rHuEPO administration and dosage.