PURPOSE: Despite being readily available and practical to administer, standardised instruments are not widely used in clinical practice. Concerns have been raised about the validity of applying such data to individuals. The aim of this qualitative study was to explore the practical difficulties of interpreting standardised HrQoL data for individual patients. METHODS: A purposive sample of 20 patients with colorectal cancer was chosen from 210 participants in a prospective HrQoL study. In an in-depth interview, individual participants were asked about their experiences and to review the accuracy of their own HrQoL data, collected over preceding months using four different instruments (FACT-C QLQ-C30/CR38 SF12 and EQ-5D). A framework qualitative analysis was used to develop emerging themes. RESULTS: A number of themes emerged from the analysis to explain why disparity arose between the patients' experiences and the questionnaire data in certain situations. These included weakly worded items that over emphasised health problems, incongruous items within scales causing unpredictable scores, insufficient levels of response causing insensitivity, and unrecognised reversal of item direction causing contradictory scores. Exogenous factors such as mood and co-morbidities also influenced HrQoL reporting. CONCLUSIONS: Data from standardised instruments can be used to measure the HrQoL of individuals in clinical practice, but the instruments used need careful selection and interpretation. Appropriate guidance linked to the themes of this study is provided.