Objective: The validity of health status questionnaires in patients attending medical rehabilitation services has been questioned. The objectives of this study were to identify problems that patients have in completing different health status questionnaires, and thus identify possible major pitfalls in interpretation of the scores. Methods: The study comprised a consecutive sample of 105 patients scheduled for inpatient rehabilitation who had completed a health status questionnaire prior to admission. They underwent a cognitive interview at admission (response rate 95.5%). Results: Patients were motivated to provide the clinic with a clear-cut picture of their illness and life situation. However, the content and response formats of the questionnaire were not specifically tailored to meet their motivation. For example, time-references predefined in the instructions were not meaningful to patients with variable symptoms. Patients' understanding of response categories was found to be ambiguous. In cases of uncertainty, patients were likely to select the "normal" middle category of response. Discussion: It is important to be aware of the problems that rehabilitation patients have in providing answers to health-related questions, because these problems are likely go unnoticed, since patients tend to provide answers even in cases of uncertainty. Instruments need to be tailored towards the motivational states, needs, cognitive capacities and subjective meanings of the respondents.