Background Informal care is increasingly common in ageing populations. However, the impact of caring responsibilities on carers’ mental health remains unclear. We used data from the HealthWise Wales (HWW) cohort to examine the impact of caregiving on the mental health of carers. Methods HWW collects demographic, lifestyle, and opinion data, which are linked to routine health-care records, from individuals aged 16 years or older who live in Wales, UK (n=21 779).