Abstract Six-month-old infants were recruited at 21 centres in the UK and Ireland and randomly assigned to receive matching iron-fortified (12.3 mg/l iron) or non-fortified (1.4 mg/l iron) formula for 9 months. Infants already receiving cow's milk continued this feed. Haematological indices and iron status were evaluated at age 6 months, 9–10 months and 15 months. Four hundred and six infants entered and 302 completed the study. There were no differences between the groups for increases in weight, head circumference or length. Significant differences between the groups were observed at 15 months for haemoglobin, serum ferritin, serum iron and total iron binding capacity. Haemoglobin levels were < 110 g/l in 33% of infants fed cow's milk compared with 13% and 11% in those receiving non-iron-fortified and iron-fortified formula respectively. The corresponding figures for serum ferritin < 10 µg/l were 43%, 22% and 6%. Follow-on formula provides an acceptable vehicle for preventing iron deficiency in this vulnerable group.