OBJECTIVE: To evaluate the linguistic validity of the Swedish version of a British health status questionnaire for Parkinson°¦s disease (PD), the 39-item PD Questionnaire (PDQ-39). METHODS: Nine consecutive non-demented PD patients (mean age: 59.2, b10.1 yrs; median Hoehn and Yahr stage of PD: III [range: II-V]) and 3 PD specialized clinicians (1 neurologist, 2 nurses) were given a copy of the Swedish version of the PDQ-39. Each questionnaire item relates to a frame question ("Due to having Parkinson's disease, how often during the last month have you...?"), appearing at the top of each questionnaire page. For each item, respondents are requested to affirm 1 of 5 response categories: "never", "occasionally", "sometimes", "often", or "always". Subjects commented on each PDQ-39 section and item as they carefully read (all subjects) and responded to (patients only) the questionnaire. All comments and responses were recorded and reviewed for accuracy at the end of each interview. RESULTS: Three aspects of the questionnaire were found particularly problematic: the frame question, the response options and 2 out of 3 items in the Social Support (SOC) subscale. Problems with the frame question related mainly to its anonymity, with patients failing to take it into consideration when reading the items, which (as several patients acknowledged) may have affected their responses. The distinction between the different response alternatives, particularly "sometimes" and "occasionally", was found unclear by five patients and one clinician. The main problem with the two items of the SOC was a double negative in the wording, which was problematic for most patients and caused some to give the opposite answer to that intended. CONCLUSIONS: This preliminary study illustrates the need for documented linguistic validity before patient-reported outcome measures can be considered suitable for use in clinical trials, research, and health economic evaluations.