OBJECTIVES: At present the costs and effects of providing informal care are often ignored in economic evaluations. This is problematic since considering informal care a 'zero cost' substitute for formal care may result in non-optimal decisions from a societal perspective, at the burden of carers. This study further investigates the validity of the CarerQol instrument, which values the full impact of informal care on caregivers. The CarerQoL consists of seven burden dimensions (CarerQol-7D) and a visual analogue scale measuring happiness (CarerQol-VAS). METHODS: A questionnaire was distributed by mail among a sample of caregivers (n=1100) via regional support centres for informal caregivers throughout the Netherlands. Construct validity (n=249) was tested with Spearman’s correlation coefficients of the CarerQol-VAS and the CarerQol-7D and two measures of subjective burden and the utility of care giving. Clinical validity was evaluated by the multivariate correlation between the CarerQol-VAS and characteristics of the caregiver, care recipient and care situation. Differences in CarerQol-VAS scores among caregivers are explained with subgroup multiple linear regression analyses and principal component analyses. RESULTS: CarerQol-VAS scores were negatively associated with burden and positively with utility of caring. The seven dimensions of the CarerQol-7D were also associated with burden and utility of caring in the expected direction, for six dimensions the associations were statistically significantly. Significant associations with CarerQol-VAS scores were found for a range of characteristics, including duration and intensity of care, relationship between carer and patient, carer education level and patient age. CONCLUSIONS: Our results further support the construct and clinical validity of the CarerQol and thus underlines the relevance of this instrument for including informal care effects in economic evaluations. The next, necessary step in the development of the CarerQol concerns the establishment of tariffs for the caring situations defined by the seven dimensions. This is the topic of coming research.