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In pharmacy practice, there is a need for valid and reliable instruments to study patient-reported outcomes. One potential candidate is a pharmaceutical therapy-related quality of life (PTRQoL) instrument. This study explored the face and content validity, including cognitive aspects of question answering of a PTRQoL instrument, translated from English to Swedish.A sample of 16 customers at Swedish community pharmacies, was asked to fill in the PTRQoL instrument while constantly reporting how they reasoned. The resulting interviews and concurrent probing, were audio-taped, transcribed verbatim and analysed using constant comparison method.The relation between the measurement and its theoretical underpinning was challenged. Respondents neglected to read the instructions, used response options in an unpredictable way, and varied in their interpretations of the items.The combination of ‘think-aloud’, retrospective probing and qualitative analysis informed on the validity of the PTRQoL instrument and was valuable in questionnaire development. The study also identified specific problems that could be relevant for other instruments probing patients' medicines-related attitudes and behaviour.