ARES is a multicenter, prospective study of the prevalence, management, and repercussions on the quality of life of anemia in renal transplant patients with a reduced renal function (creatinine clearance according to Cockcroft-Gault: </=60 and >15 mL/min). The frequency of factor deficiency and its relationship with anemia were analyzed at the baseline time of the study. Of the 500 patients included in the main study, valid data were available for iron metabolism in n = 419 microg/dL; folic acid, n = 205 ng/mL; and vitamin B12, n = 210 pg/mL. Anemia was defined as hemoglobin </=13 g/dL (men) or </=12 g/dL (women) and/or use of erythropoietin (EPO). Anemic patients (59.4%) had less sideremia (73.4 vs 81.2 microg/dL; P = .008), but no significant differences were observed for transferrin saturation index (25.9% vs 25.5%), ferritin (167 vs 171 ng/mL), iron insufficiency (26.5% vs 36.2%), pronounced ferropenia (20.4% vs 20.1%), folic acid (7.5 vs 6.6 ng/mL), or vitamin B12 (486 vs 530 pg/mL). Treatment with oral or intravenous iron was much more frequent in anemic patients (31.6% vs 9.9%; P < .001). The logistic regression analysis of factors associated with anemia revealed that renal function and the use of angiotensin-converting enzyme (ACE) inhibitors were significant but not the degree of iron deficiency. In conclusion, iron deficiency in renal transplant patients with chronic nephropathy is frequent and insufficiently treated. Although it may be an aggravating factor, it was not shown to be a determining factor for the presence or absence of anemia in the patients as a group.