Background and purpose The Knee Injury and Osteoarthritis Outcome Score (KOOS) is distinguished from other knee-specific measures by the inclusion of separate scales for evaluation of activities of daily living, sports and recreation function, and knee-related quality of life, with presentation of separate subscale scores as a profile. However, its applicability in children has not been established. In this study, we examined how well the KOOS could be understood in a cohort of children with knee injury, with a view to preparing a pediatric version (KOOS-Child). Material and methods A trained researcher conducted cognitive interviews with 34 Swedish children who had symptomatic knee injuries (either primary or repeated). They were 10-16 years of age, and were selected to allow for equal group representation of age and sex. All the interviews were recorded. 4 researchers analyzed the data and modified the original KOOS questionnaire. Results Many children (n = 14) had difficulty in tracking items based on the time frame and an equivalent number of children had trouble in understanding several terms. Mapping errors resulted from misinterpretation of items and from design issues related to the item such as double-barreled format. Most children understood how to use the 5-point Likert response scale. Many children found the instructions confusing from both a lexical and a formatting point of view. Overall, most children found that several items were irrelevant. Interpretation The original KOOS is not well understood by children. Modifications related to comprehension, mapping of responses, and jargon in the KOOS were made based on qualitative feedback from the children.