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Purpose:Epidemiological studies increasingly include DNA sampling in their protocols, but participation may vary by demographic, psychological, or health characteristics. This analysis explored sociodemographic (age, gender, religion, and education), health-related, and psychological predictors of participation in genetic data collection in two large epidemiological studies: the Whitehall II cohort and the English Longitudinal Study of Ageing. DNA consent in both studies was not obtained on initial enrollment, but after long-standing participation in the study.Methods:Study participants who accepted or declined DNA sampling were compared.Results:Very few participants declined DNA sampling; 1.6% in Whitehall II and 1.0% in English Longitudinal Study of Ageing. In both cohorts, participants who declined were more likely to be female, nonwhite, and involved in religious activities; although only the ethnic effect consistently survived multivariate analyses. Decliners also felt less in control of events in their lives than those who consented, and this remained significant in multivariate analysis in the English Longitudinal Study of Ageing sample.Conclusion:Consent to DNA sampling was very high in two established UK cohort studies. Differences between the subset of the population unwilling to provide DNA samples and those who consented were modest, although this may be due to the highly selected populations in these cohorts. However, our findings suggest that a greater proportion of decliners would create an important source of selection bias.Genet Med 2012:14(2):201–206