A matched case-control study was carried out to identify risk factors for highly pathogenic avian influenza A virus (subtype H5N I) infection in commercial chickens in Bangladesh. A total of 33 commercial farms diagnosed with H5N I before September 9, 2007, were enrolled as cases, and 99 geographically matched unaffected farms were enrolled as control farms. Farm data were collected using a pretested questionnaire, and analysed by matched-pair analysis and multivariate conditional logistic regression. Two factors independently and positively associated with H5N I infection remained in the final model. They were 'farm accessible to feral and wild animals' (odds ratio [OR] 5.71, 95 per cent confidence interval [Cl] 1.81 to 18.0, P=0.003) and 'footbath at entry to farm/shed' (OR 4.93, 95 per cent Cl 1.61 to 15.1, P=0005). The use of a designated vehicle for sending eggs to a vendor or market appeared to be a protective factor (OR 0.14, 95 per cent Cl 002 to 0.88, P=0.036).