We evaluated the adverse effect of asymptomatic Helicobacter pylori infection in children on the response to Fe supplementation. One hundred and sixty-nine children aged 1–10 years from the urban poor community underwent a [13C]urea breath test for H. pylori and haematological tests at admission and after 8 weeks. Both H. pylori-positive and -negative children were randomly assigned to receive ferrous fumarate syrup (20 mg elemental Fe twice daily) or placebo for 8 weeks and a single dose of vitamin A (33,000μg). Admission findings were compared between H. pylori-positive and -negative children. Response to Fe was compared between Fe-supplemented H. pylori-positive and -negative children. Seventy-nine per cent of the children were aged 1–5 years and half of them were boys. In eighty-five H. pylori-positive and eighty-four H. pylori-negative children, the differences in mean Hb (112 (sd 12·6) v. 113 (sd 12·0) g/l), haematocrit (34 (sd 3·5) v. 35 (sd 3·2) %) and ferritin (23·8 v. 21·0 μg/l) were similar. After 8 weeks of Fe supplementation, mean Hb was 5·3 g/l more (95 % CI 1·59, 9·0) and haematocrit was 1·4 % more (95 % CI 0·2, 2·6) in H. pylori-negative (n 44) compared with H. pylori-positive (n 42) children. Mean ferritin was similar at admission and improved in both H. pylori-positive and -negative children. Asymptomatic H. pylori infection was not associated with higher rates of anaemia or Fe deficiency in children, but had a significant adverse effect on response to Fe therapy. However, this result is based on exploratory analysis and needs confirmation.