Background and Purpose. This study examined the reliability, validity, and responsiveness to change of measurements obtained with a 66-item version of the Gross Motor Function Measure (GMFM-66) developed using Rasch analysis. Subjects and Methods. The validity of measurements obtained with the GMFM-66 was assessed by examining the hierarchy of items and the GMFM-66 scores for different groups of children from a stratified random community-based sample of 537 children with cerebral palsy (CP). A subset of 228 children who had been reassessed at 12 months was used to test the hypothesis that children who are young (<5 years of age) and have “mild” CP will demonstrate greater change in GMFM-66 scores than children who are older (≥5 years of age) and whose CP is more severe. Data from an additional 19 children with CP who were assessed twice, one week apart, were used to examine test-retest reliability. Results. The overall changes in GMFM-66 scores over 12 months and a time × severity × age interaction supported our hypotheses. Test-retest reliability was high (intraclass correlation coefficient=.99). Conclusion and Discussion. This study demonstrated that the GMFM-66 has good psychometric properties. By providing a hierarchical structure and interval scaling, the GMFM-66 can provide a better understanding of motor development for children with CP than the 88 item GMFM and can improve the scoring and interpretation of data obtained with the GMFM.