Intermittent iron supplementation has been suggested as a replacement for daily iron supplements for reducing anemia in developing countries. The effects of once weekly and daily iron supplementation on hemoglobin (Hb), serum ferritin (SF), prevalence of anemia, weight and height are compared in this study. Primary schoolchildren (n = 397) from two selected schools in the Hat Yai rural area, southern Thailand, were recruited in 1999. All children received Albendazole and then randomly received ferrous sulfate (300 mg/tablet) either daily or weekly, or a placebo for 16 wk. The average increase in Hb was not significantly different between the daily (mean +/- SD; 6.5 +/- 6.0 g/L) and weekly (5.7 +/- 6.3 g/L) groups. However, the average increase in SF was greater (P < 0.01) in the daily (mean +/- SD; 39.8 +/- 30.3 microg/L) than the weekly (13.4 +/- 17.3 microg/L) group. All cases of iron deficiency anemia were abolished in both daily and weekly groups, whereas no reduction in prevalence occurred in the placebo group. Height gain was greater in children who received weekly (mean +/- SD; 2.6 +/- 0.9 cm) than in those who received daily iron (mean +/- SD; 2.3 +/- 0.8 cm), (P < 0.01). Weight gain, weight-for-age and height-for-age were not significantly different among the intervention groups. It is concluded that a weekly iron dose is more effective than a daily dose in height gain but not in hematological improvement over 16 wk of supplementation.