BACKGROUND: The purpose of this study was to determine the extent to which summary index scores from the short form Parkinson's Disease Questionnaire (PDQ-8) replicate those from the parent form (PDQ-39) in a longitudinal study. METHODS: Longitudinal data gained from the PD-MED trial were examined (n = 1867), to determine the extent the PDQ-8 replicates results from the PDQ-39 at baseline and follow up. The sensitivity to change of the PDQ-8 was also compared with that of the PDQ-39. Finally, results on the two measures were compared with those from the Hoehn and Yahr (HY) clinical staging scale. RESULTS: Results of the Single Index summary score gained from the PDQ-8 were found to closely replicate those gained from the PDQ-39 at each of the three time points. Furthermore at each time point the intraclass correlation coefficient between the two measures was very high (ICC range 0.93-0.96). Similarly, the two measures gave very similar accounts of change (e.g. from baseline to follow up at one year effect sizes were 0.18 for the single index calculated using the PDQ-39, and 0.09 when calculated using the PDQ-8). Similar levels of correlation were found between the two indices when correlated with the HY scale. CONCLUSIONS: The PDQ-8 closely replicates results gained from the PDQ-39 when calculating single indices. In instances where a single summary score of the impact of PD on self-reported quality of life is needed, it is likely the PDQ-8 will provide reliable and accurate information.