Recent studies have shown that the serum transferrin receptor is a sensitive, quantitative measure of tissue iron deficiency. This study was undertaken to determine the serum transferrin receptor's ability to distinguish iron-deficiency anemia from the anemia of chronic inflammation and to identify iron deficiency in patients with liver disease. The mean transferrin receptor level in 17 normal controls was 5.36 +/- 0.82 mg/L compared with 13.91 +/- 4.63 mg/L in 17 patients with iron-deficiency anemia (p less than 0.001). The mean serum receptor level was normal in all 20 patients with acute infection, including five with acute hepatitis, and was also normal in 8 of 10 anemic patients with chronic liver disease. Receptor levels were in the normal range in all but 4 of 41 patients with anemia of chronic disease. We conclude that unlike serum ferritin levels, which are disproportionately elevated in relation to iron stores in patients with inflammation or liver disease, the serum transferrin receptor level is not affected by these disorders and is therefore a reliable laboratory index of iron deficiency anemia.