OBJECTIVE To determine the value of serum ferritin, mean cell volume, transferrin saturation, and free erythrocyte protoporphyrin in the diagnosis of iron-deficiency anemia in the elderly. METHODS We prospectively studied consecutive eligible and consenting anemic patients over the age of 65 years, who underwent blood tests and bone marrow aspiration. The study consisted of 259 inpatients and outpatients at two community hospitals in whom a complete blood count processed by the hospital laboratory demonstrated previously undiagnosed anemia (men: hemoglobin level less than 12 g/dL; women: hemoglobin level less than 11.0 g/dL). RESULTS Thirty-six percent of our patients had no demonstrable marrow iron and were classified as being iron-deficient. The serum ferritin was the best test for distinguishing those with iron deficiency from those who were not iron-deficient. No other test added clinically important information. The likelihood ratios associated with the serum ferritin level were as follows: greater than 100 micrograms/L, 0.13; greater than 45 micrograms/L but less than or equal to 100 micrograms/L, 0.46; greater than 18 micrograms/L but less than or equal to 45 micrograms/L, 3.12; and less than or equal to 18 micrograms/L, 41.47. These results indicate that values up to 45 micrograms/L increase the likelihood of iron deficiency, whereas values over 45 micrograms/L decrease the likelihood of iron deficiency. Seventy-two percent of those who were not iron-deficient had serum ferritin values greater than 100 micrograms/L, and in populations with a prevalence of iron deficiency of less than 40%, values of greater than 100 micrograms/L reduce the probability of iron deficiency to under 10%. Fifty-five percent of the iron-deficient patients had serum ferritin values of less than 18 micrograms/L, and in populations with a prevalence of iron deficiency of greater than 20%, values of less than 18 micrograms/L increase the probability of iron deficiency to over 95%.