This study is a randomised double blind controlled cluster trial of the effectiveness of multiple micronutrient supplements given by teachers to schoolchildren on improving their growth, nutritional status and educational achievement. The primary outcome measures are: gains in weight and height, haemoglobin concentration, cognitive function and educational achievement. Secondary outcomes are other anthropometric measurements and a decrease in body arsenic deposits. One hundred schools in Meherpur District, Bangladesh will take part in this study. Participation will be on the basis of informed consent. All pupils will be dewormed twice a year using a single dose of 400 mg albendazole; this will promote the bioavailability of the micronutrient supplements. The schools will be randomly allocated into two groups. In one group of 50 schools all children will be given by their teachers a single tablet of multiple micronutrients on every other school day each week, so for a total of 3 days a week or about 125 school days a year, depending on school and national holidays. This supplement will make a substantial contribution over a period of one year to each child's recommended intake of iron, iodine, zinc, vitamin A, vitamin B12, folate and four B-group vitamins. Some of these micronutrients, such as vitamin A and iron, can be stored in the body, in addition to meeting current nutritional requirements. Children in the other 50 schools will receive a tablet identical in weight, shape, size and appearance that contains the same four B-group vitamins as the multiple micronutrient supplement. The B-group vitamins are not expected to have a major effect on any of the outcome measures, but may provide some benefit to children in the comparison group. Both supplements are formulated according to UK and US pharmacopoeias by Acme Pharmaceuticals and are being purchased by Save the Children. Both supplements will be given free on three days a week to an estimated 30,000 pupils in all 100 study schools for a period of two school years. The subjects for study will be up to 30 randomly selected children in class 2 in the age range 7 - 8 years in all 100 schools. An equal number of boys and girls will be selected. The actual number will depend on enrolment in class 2 in each school. A baseline survey will be done before the supplements are started to measure the following: weight, height and other anthropometric measurements to assess growth; the prevalence of intestinal worm infections in a 20% sub-sample; the haemoglobin concentration in a drop of blood taken from a finger prick from a sub-sample of about 66% of all children; the concentration of arsenic in hair in a sub-sample of about 20% of all children; tests of cognitive function including concentration in all children; and tests of educational achievement in mathematics and language in all children. These measurements, analyses and assessments will be repeated two years later when the cohort of up to 1,500 children in each study group are in class 4. The magnitude and statistical significance of differences in the mean differences in outcome values between study groups will indicate the impact of the multiple micronutrient supplement in comparison with B-group vitamins alone.