PURPOSE The purpose of this article was to determine the prevalence of iron, vitamin B12, and folate deficiency and to evaluate the erythropoietin (EPO) response to anemia in a cohort of long-term intensive care unit (ICU) patients. MATERIALS AND METHODS All patients admitted to three academic medical center multidisciplinary ICUs were screened for eligibility into a randomized trial of EPO for the treatment of ICU anemia. On their second or third ICU day, patients enrolled in this trial had EPO levels drawn and were screened for iron, B12, and folate deficiency. Weekly EPO levels were obtained throughout patients' ICU stay. RESULTS A total of 184 patients were screened for iron, B12, and folate deficiency. Sixteen patients (9%) were iron deficient by study criteria, 4 (2%) were B12 deficient, and 4 (2%) were folate deficient. Mean hemoglobin and reticulocyte percents of the remaining 160 patients were 10.3 +/- 1.2 g/dL and 1.66 +/- 1.09%, respectively. In most patients, serum iron and total iron binding capacity levels were very low, whereas ferritin levels were very high. Mean and median day 2 EPO levels were 35.2 +/- 35.6 mIU/mL and 22.7 mIU/mL, respectively (normal = 4.2-27.8). Serial EPO levels in most persistently anemic patients remained within the normal range. CONCLUSIONS In this cohort, screening for iron, B12, and folate deficiency identified potentially correctable abnormalities in more than 13% of patients and should be considered in those who are anticipated to have long ICU stays. Even at an early point of critical illness, most patients had iron studies consistent with anemia of chronic disease (ACD), as well as a blunted EPO response that may contribute to this ACD-like anemia of critical illness.