BACKGROUND The increased use of computer-based records has facilitated linkage of routine data with that obtained for research. When children are involved, parental consent for linkage is usually required. The Millennium Cohort Study, of 18,819 UK babies born in 2000-02, over-sampled families from disadvantaged and ethnic wards, providing the opportunity to investigate factors associated with mother's consent to access her child's birth records. METHODS Factors considered included ward type and mother's socioeconomic status, ethnicity, education, age, and language. Logistic regression was used to investigate the relationship of these factors with consent. RESULTS Consent for linkage to birth register and/or hospital maternity data was obtained from 92% of the cohort mothers. The proportions consenting differed according to the mother's country of residence, age, and education, with consent being less likely among minority ethnic group mothers, lone parents, and those with higher degrees or no qualifications. Where interviews had been translated, consent was significantly less likely if the interpreter was a male. CONCLUSION A large proportion of mothers who were interviewed gave permission for linkage. However, there were some groups who were less likely to do so, particularly those from minority ethnic groups. These sources of non-consent bias should be taken into account when analysing linked data from socially and ethnically mixed populations. Efforts should be made to understand the reasons for non-consent, which in turn will help determine the best ways to encourage more mothers to consent in future.