The objectives of this three-phased study were to design and evaluate the usability of a computerized questionnaire, The Children's Computerized Physical Activity Reporter, designed with and for middle school children's self-report of physical activity. Study design was qualitative, descriptive, and collaborative, framed in a usability engineering model, with 22 participating children (grades 6-8; mean age, 12.5 years; range, 11-15 years) of three ethnic backgrounds. In Phase 1, children's understanding of physical activity and needs for reporting were determined, which were then translated in Phase 2 to the design features and content of the questionnaire; content validity, readability, and algorithm reliability were completed. Phase 3 involved children's evaluation of the questionnaire's usability (ease of use, efficiency, and aesthetics). The children all liked the questionnaire but identified several usability issues within instructions and reports. Working collaboratively with children was highly effective in ascertaining their understanding of physical activity and their self-reporting needs. Thus, the questionnaire's design was created from children's understanding of physical activity and their needs for recalling activities. The development of the questionnaire and its usability evaluation contribute to understanding children's physical activity and to the importance of designing for usability. Additional research is needed to ascertain reliability and validity of data derived from its use and to explore its usefulness in clinical or research venues.