Ethical guidelines in the United Kingdom require written consent from participants in epidemiologic studies for follow-up or review of medical records. This may cause bias in samples used for follow-up or medical record review. The authors analyzed data from seven general population surveys conducted in the United Kingdom (1996-2002), to which over 25,000 people responded. Associations of age, gender, and symptom under investigation with consent to follow-up and consent to review of medical records were examined. Consent to follow-up was approximately 75-95% among survey responders under age 50 years but fell among older people, particularly females. Consent to follow-up was also higher among responders who had the symptom under investigation (pooled odds ratio = 1.61, 95% confidence interval: 1.36, 1.92). Consent to review of medical records followed a similar pattern. Patterns of consent were relatively consistent and represented a high proportion of responders. Males, younger people, and subjects reporting the symptom under investigation were more likely to give consent, and these groups may be overrepresented in follow-up samples or reviews of medical records. Although consent is high among responders, the additive effect of nonresponse and nonconsent can substantially reduce sample size and should be taken into account in epidemiologic study planning.