This study examined the variability in the measurement of the acetabular index (AI) in normal and dysplastic hips, both before and after reduction. This variability for dysplastic hips is greater than that of normal hips. The variability is greater before an open or closed reduction than after reduction, and the variability after an open reduction is nearly 3 times greater than after a closed reduction. The 95% confidence interval of the AI is 10.1° intraobserver and 21.9° interobserver for all hips. The AI is most accurate in the situation in which it is most useful, after a closed reduction of a dysplastic hip. A 95% confidence interval of 5.1° in this selected population supports the use of the AI for monitoring acetabular remodeling after closed reductions in accordance with previous clinical recommendations.