An evaluation of 26 surviving outpatient lung transplant recipients at one center showed that 65% (17/26) had significant anemia (hemoglobin < 11 g/L for women, < 14 g/dl for men) at a median follow-up of 13.5 months after transplantation (range, 1-41 months). There were 14 men and 12 women with a mean age of 45.1 years (range, 23.1-66.7 years). Fifteen had a double allograft and 11 had a single allograft. Anemia was normochromic and normocytic/macrocytic with a tendency to anisocytosis, with normal reticulocyte counts. Iron deficiency (transferrin saturation < 20%) was found in 35% (6/17) of anemic patients, and two of them also had ferritin levels < 15 micrograms/L. In addition, vitamin B12 was decreased in 1 patient. Folate levels were all normal. Erythropoietin levels were significantly decreased in anemic lung transplant recipients as compared with nontransplanted iron-deficient anemic patients (median, 1 mU/ml, range 1-41 mU/ml, vs. 53 mU/ml, 15-88 mU/ml; P < 0.05). In nonanemic lung transplant recipients, erythropoietin levels were decreased too, as compared with normal controls (median, 2 mU/ml, range 1-21 mU/ml, vs. 5 mU/ml, 3-32 mU/ml; P < 0.05). Investigation of peripheral stem cells in 9 patients showed normal stimulation of erythroids (burst-forming unit, erythroid; median, 573 cells/ml; range, 128-1898 cells/ml) independent of erythropoietin concentrations. Analysis of putative prognostic factors, such as age, surgical procedure (double vs. single lung allograft), indication for transplantation, time after transplantation, infection status, presence of bronchiolitis obliterans, immunosuppression (+/- azathioprine), serum creatinine, creatinine clearance, hypertension, and arterial partial pressure of oxygen, did not demonstrate any difference in erythropoietin concentrations. Only the sex variable revealed a trend to higher levels in women than in men (median, 4 mU/ml, range 1-41 mU/ml, vs. 1 mU/ml, 1-16 mU/ml; P > 0.05). The causes for low erythropoietin levels are not quite understood yet; however, they offer a rationale for the treatment of chronic anemia with recombinant human erythropoietin.