Radiologic measures of migration percentage (MP) and acetabular index (AI) taken from plain radiographs of the pelvis are the most commonly used tools for determining hip displacement and management options in children with cerebral palsy and spastic hip disease. This study determined interrater and intrarater reliability of MP and AI on pelvic radiographs chosen to represent a wide range of age (11 months to 8 years 5 months), MP (0%–56%), and AI (9°–33°). The study demonstrated that an experienced rater would be expected to measure MP on a single radiograph to within ±5.8% of the true value and a change in MP between two radiographs taken at different times to within ±8.3% of the true value. Similarly for AI, the measurement error for a typical rater would be within ±2.6°on a single reading and ±3.7° if recording change between two occasions. The authors believe that the results indicating true change are acceptable in clinical practice, provided treatment decisions are based on a series of radiographs taken at 6-month intervals, methods and training are standardized, and consistent raters are used.