OBJECTIVES: The PDQ-Carer is a 29 item measure of health related quality of life for use with carers of people with Parkinson’s disease. The original development of the instrument identified four sub domains measured by the instrument. This study examined the possibility that the four domains could reasonably be summed to also provide a single summary index score. METHODS: The PDQ-Carer was administered in a postal survey of patients and carers registered with local branches of the Parkinson's Disease Society of Great Britain. Data from the four dimensions of the PDQ-Carer was subjected to higher order factor analyses. This produced a single factor suggesting that a single index can be calculated from the measure. Content validity of the measure was assessed by correlating results with the ‘General Health Perceptions’ scale of the SF-36, a generic measure of health status. RESULTS: A total of 236 carer questionnaires were returned, a response rate of 60.9%. The mean age of the sample was 68.2 years (SD 9.49; range 25-89 years); 63.5% females, 21.3% males. Higher order principle components factor analysis produced one factor, accounting for 85.5% of the variance. Consequently it was decided that the scores of the four domains could be summed to produce a single index figure. The psychometric properties of this index were explored using reliability tests and tests of construct validity. The newly derived single index was found to be both internally reliable (alpha=0.94) and supported by construct validity (correlation with GHP =0.50, p<0.001). CONCLUSIONS: The analysis undertaken here indicates that data from the PDQ-Carer can be presented in summary form. The index will provide an overall indication of the impact of caring. Furthermore, the single index reduces the number of statistical comparisons, and hence the role of chance, when exploring data from the PDQ-Carer.