The effect of daily rather than weeky iron supplementation was compared in women who were 8-24 wk pregnant. One group (n = 68) received 60 mg Fe/d, the second group (n = 71) received 120 mg Fe/wk, given at once. Supplementation lasted 11.3 wk on average, depending on gestational date at entry, and was not supervised. Hemoglobin increased in both groups (P < 0.001); serum ferritin did not change significantly. There was no significant difference between groups for changes in hemoglobin and serum ferritin. In a subgroup of women with a hemoglobin concentration < 110 g/L at baseline (n = 45 daily; n = 54 weekly) no significant within-group changes occurred in serum ferritin, but the change in the daily group was 4.1 mu,g/L higher than in the weekly group (P = 0.049). Compliance, as indicated by two positive stool tests, was approximate to 54.3% in the daily group and 62.2% in the weekly group. We conclude that for the complete sample of subjects, the treatment effect of daily compared with weekly supplementation was similar under conditions resembling a normal antenatal care program.