The usefulness of serum ferritin levels in assessing iron stores in pregnant women before and after supplementation with iron was studied. One hundred thirty-five healthy pregnant women between 22 to 28 wk were randomly allotted to daily dose regimes of 60, 120, or 240 mg of ferrous sulphate. The tablets were given after meals under strict personal supervision. Before supplementation, iron deficiency (ferritin level less than 10 micrograms/L) was present in 54.8% of the pregnant women, compared to an incidence of 17.8% when assessed by serum iron concentration of less than 50 micrograms/dl. The mean ferritin level of pregnant women was 14.15 micrograms/L and was less than one-half that of healthy single women and one-sixth of that of healthy males. Supplementation with oral iron for 12 wk produced an increase in ferritin levels in all the groups, but significant increases were seen only in women given 120 and 240 mg of ferrous sulphate with or without folic acid. However, there were no differences in final Hb levels among the supplemented groups.