The use of hemoglobin as a predictor of response to iron therapy, for screening, and for prevalence estimates was studied. An Fe supplementation trial was performed in Quito, Ecuador, in which 412 pregnant women were randomly assigned to treatment and control groups. Women in the treatment group received 390 mg ferrous sulfate/d for 2 mo. The prevalence of Fe deficiency as defined by response to therapy was found to be 60.8%. Sensitivity and specificity were calculated at various cutoff points of hemoglobin. The estimates of sensitivity and specificity allow for the use of hemoglobin in screening for Fe deficiency anemia and in the estimation of the prevalence in populations with characteristics similar to those found in the sample of pregnant women in Quito. Hemoglobin was shown to be a good predictor of response to Fe treatment and a good estimate of prevalence of Fe deficiency when prevalence is high.