The sensitivity and predictive value of serum ferritin (SF) and free erythrocyte protoporphyrin (FEP) for iron deficiency (ID) was evaluated by studying 272 subjects with uncomplicated ID (174 with anemia and 98 without) in whom diagnosis was confirmed by the response to iron supplementation. Overall, the sensitivity, at 95% specificity, was 82% (79% in women, 94% in men) for SF and 61% (60% in women, 65% in men) for FEP. The sensitivity varied as a function of hemoglobin values, dropping from over 90% for both tests in the case of severe anemia, to approximately 70% for SF and less than 50% for FEP in the absence of anemia. The predictive value decreases more sharply for FEP than for SF with increasing hemoglobin levels. It is concluded that SF is preferable to FEP for the detection of ID, particularly in the absence of anemia. However, owing to the unsatisfactory predictive value at low prevalence, SF should be used as a screening test for ID without anemia only when the prevalence is at least 20%.