objectives: to describe informal caregivers' characteristics, quantify the effect of caregiving activities on caregivers' health, and determine whether this effect differs by sex. methods: a descriptive study was carried out based on the basque health survey 2002. physical and mental health, use of health services, social health and health related quality of life (hrql) indicators were compared in caregivers (n = 836) and non-caregivers (n = 5,706). age-adjusted logistic regression models were calculated to assess the association between the caregiver and effects on health by sex, socioeconomic position and occupational status. results: female caregivers showed worse results in 6 out of 10 indicators compared with non-caregiver women, whereas male caregivers showed worse results in only four indicators compared with non-caregiving men. the associations between caregiving (caregiving vs. non-caregiving) and health results were stronger in women than in men, except in social health. when caregiving burden intensity was considered, greatly burdened men showed a similar or higher risk of poor hrql (physical in men: or = 3.0, ci95%: 1.4-6.3; women: or = 2.3, ci95%: 1.5-3.5; mental in men: or = 2.5, ci95%: 1.4-4.3; women: or = 2.5, ci95%: 1.7-3.7) and low social support (affective support in men: or = 2.6, ci95%: 1.5-4.6; women: or = 1.5, ci95%: 1.0-2.3). however, the risk of sedentariness, lack of sleep, greater use of health services and mental ill-health remained higher for women caregivers. conclusions: caregiving damages the health of informal caregivers but the risks for female caregivers are higher due to greater intensity of caregiver burden. as men's caregiving burden increases, gender inequalities decrease or invert.