A recent meta-analysis of 14 clinical trials indicated that daily compared with intermittent iron supplementation resulted in significantly greater hematological improvement in pregnant women. No such definitive beneficial effect was demonstrated in preschool children. We compared the efficacy of daily and twice weekly iron supplementation for 6 wk under supervised and unsupervised conditions in the treatment of mild and moderate anemia [hemoglobin (Hb) 50-109 g/L] in children aged 2-59 mo living in a malaria-endemic area of western Kenya. The study was a cluster-randomized trial using a factorial design; participants were aware of the treatment assigned. All children (n = 1049) were administered a single dose of sulfadoxine-pyrimethamine at enrollment followed by 6 wk of daily supervised iron supplementation [3-6 mg/(kg.d)], twice weekly supervised iron supplementation [6-12 mg/(kg.wk)], daily unsupervised iron supplementation, or twice weekly unsupervised iron supplementation. In the supervised groups, Hb concentrations at 6 and 12 wk (6 wk postsupplementation) were significantly higher in children given iron daily rather than twice weekly [mean (95% CI) difference at 6-wk: 4.2 g/L (2.1, 6.4); 12-wk: 4.4 g/L (1.8, 7.0)]. Among the unsupervised groups, Hb concentrations were not different at 6 wk [mean (95% CI) difference: 0.86 g/L (-1.4, 3.1)], but significantly higher at 12 wk for those assigned daily iron [mean (95% CI) difference: 3.4 g/L (0.79, 6.0), P = 0.02]. In this malarious area and after initial antimalarial treatment, 6 wk of daily iron supplementation results in better hematological responses than twice weekly iron supplementation in the treatment of anemia in preschool children, regardless of whether adherence can be ensured.