In order to test the impact of Giardia and geohelminthic infection on infant growth faltering in Bangladesh, a randomised double-blind placebo controlled intervention of 36 weeks' duration was conducted in a rural community located 40 km northwest of Dhaka. Infants aged between 3 and 15 months were randomly assigned to either anti-Giardia and anthelminthic treatment, anti-Giardia treatment only, or a control. Weight and supine length were recorded every 4 weeks. Every 12 weeks intestinal permeability (lactulose/mannitol ratio), haemoglobin, plasma albumin, alpha-1-acid glycoprotein, IgG and Giardia-specific IgM (GSIgM) and eggs of the three common geohelminths and G. intestinalis cysts were determined. Data on 222 fully compliant infants were analysed. No significant differences in intestinal permeability, biochemical or anthropometric variables were found between the intervention groups, although there were associations between improvement in small intestinal mucosal function and better weight-for-age and weight-for-height (length) Z-scores. GSIgM titres indicated high endemicity with rapid re-infection of Giardia among infants; over 95% of infants were positive throughout the study, whereas the stool examination showed very few infants with either geohelminth eggs or Giardia cysts (ClinicalTrials.gov Identifier: NCT00607074).