Background Providing informal care has been linked with poor health but has not previously been studied across a whole population. We aimed to study the association between informal care provision and self-reported poor health.

Method We used data from the UK 2001 Census. The relationship between informal caregiving and poor health was modelled using logistic regression, adjusting for age, sex, marital status, ethnicity, economic activity and educational attainment.

Results We included 44 465 833 individuals free from permanent sickness or disability. 5 451 902 (12.3%) participants reported providing informal care to another person. There was an association between provision of informal caregiving and self-reported poor health; OR 1.100, 95% CI 1.096 to 1.103. This association remained after adjustment for age, sex, ethnic group, marital status, economic activity and educational attainment. The association also increased with the amount of care provided (hours per week).

Conclusions Around one in eight of the UK population reports that he or she is an informal caregiver. This activity is associated with poor health, particularly in those providing over 20 h care per week.