Thirty patients with anemia of various causes received packed red cell transfusions. To evaluate the influence of blood transfusion on the serum levels of different nutrients, serum ferritin, folate, and cobalamin levels were determined before and 2, 2 to 4, 5 to 7, 8 to 10, and 11 to 14 days after blood transfusion. No significant change was found in these levels before or at any time after blood transfusion. Blood transfusion exerted little effect on the serum levels of ferritin, cobalamin, or folate. Moreover, if the blood for testing was drawn 1 to 14 days after transfusion, nearly all cases of nutritional deficiency anemia could have been diagnosed without the influence of blood transfusion.