The Activities Scale for Kids (ASK) is a self-report measure of childhood physical disability, that has excellent reliability (ICC = 0.97). The purpose of this study was to assess further the ASK's measurement properties. ASK questionnaires were completed by 200 children with musculoskeletal limitations (mean age, 10.1 years). Rasch analyses confirmed that all items measured the same construct and supported aggregation of a summary score. Validity of the ASK was demonstrated by a correlation of 0.81 (P<0.0001) with parent-reported Childhood Health Assessment Questionnaire scores; a significant difference in scores according to clinicians' global ratings of disability (P<0.0001), and a correlation of 0.92 (P<0.0001) with clinician-observation. The ASK showed minimal ceiling effects, no floor effects, and changed by 1.73 standard deviation units after clinically important change. The ASK is a valid and responsive outcome measure that permits 5- to 15-year-old children physical functioning in the community to be assessed accurately by mail. The quality of this measure will enable clinical studies to measure outcome not only in a way that is relevant to patients, but also in a way that is sensitive to small amounts of change and is practical and inexpensive.