The purpose of this study was to demonstrate the use of Latent Growth Modeling (LGM) as a method for estimating reliability of Curriculum-Based Measurement (CBM) progress-monitoring data. The LGM approach permits the error associated with each measure to differ at each time point, thus providing an alternative method for examining of the reliability of CBM reading aloud data over repeated measurements. The analysis revealed that the reliability of CBM data was not a fixed property of the measure, but it changed with time. The study demonstrates the need to consider reliability in new ways with respect to the use of CBM data as repeated measures. (Contains 5 figures and 3 tables.)