A double-blind clinical trial was conducted among 9- to 11-year-old children in sixteen schools in the Chon Buri province of Thailand to assess the effects of an iron supplement combined with an anthelminthic agent (i.e. albendazole). In addition to the albendazole, Fe or placebo tablets were distributed to 2268 children enrolled in grades three to five without knowledge of the Fe status of the children. Criteria for case inclusion were: (a) absence of A E Bart's or haemoglobin (Hb) H disease, (b) absence of abnormal Hb EE, and (c) age, 108–144 months. The results showed a significant improvement in the Fe status of the children after 16 weeks of treatment. The increments were: Hb from 124 to 128 g/l, serum ferritin from 34.54 to 104.72 μg/l, transferrin saturation from 24.09 to 35.05%; free erythrocyte protoporphyrin decreased from 444.7 to 281.4 μg/l erythrocytes. These changes were significantly greater than in the control group that received only the anthelminthic agent. However, the administration of albendazole only also resulted in significant changes in the same Fe indicators.