Background: Quality of life (QoL) is considered to be an indispensable outcome measure of curative and palliative treatment. However, QoL research often yields findings that raise questions about what QoL measurement instruments actually assess and how the scores should be interpreted. Objective: To investigate how patients interpret and respond to questions on the EORTC-QLQ-C30 over time and to find explanations to account for counterintuitive findings in QoL measurement. Methods: Qualitative investigation was made of the response behaviour of small-cell lung cancer patients (n = 23) in the measurement of QoL with the European Organization for Research and Treatment of Cancer Core Quality of Life Questionnaire (EORTC QLQ-C30). Focus was on physical functioning (PF, items 1 to 5), role functioning (RF, items 6 and 7), global health and QoL rating (GH/QOL, items 29 and 30). Interviews were held at four points: at the start of the chemotherapy, 4 weeks later, at the end, and 6 weeks after the end of chemotherapy. Patients were asked to 'think aloud' when filling in the questionnaire. Results: Patients used various response strategies when answering questions about problems and limitations in functioning, which impacted the accuracy of the scale. Patients had scores suggesting they were less limited than they actually were by taking the wording of questions literally, by guessing their functioning in activities that they did not perform, and by ignoring or excluding certain activities that they could not perform. Conclusion: Terminally ill patients evaluate their functioning in terms of what they perceive to be normal under the circumstances. Their answers can be interpreted in terms of change in the appraisal process (Rapkin and Schwartz 2004; Health and Quality of Life Outcomes, 2, 14). More care should be taken in assessing the quality of a set of questions about physical and role functioning. © 2008 The Author(s).