Curriculum-based measurement of reading (CBM-R) is used in research and practice to estimate the level and trend of student achievement. Although there is limited empirical or psychometric support to guide CBM-R progress monitoring practices, derived trend estimates are used to inform a variety of educational decisions including evaluations of program effects and a student's response to reading interventions. This study evaluated the quality of trend estimates derived from CBM-R progress monitoring data and evaluated the external validity of the results from previous simulation studies. Forty second-grade students participated in a 6-week daily schedule of CBM-R progress monitoring. The data were used to evaluate the validity, reliability, precision, and general quality of trend estimates. The results suggested that progress monitoring outcomes were more reliable, valid, and precise as the duration of progress monitoring and number of data points increased; however, the quality of estimates was only marginal after 6 weeks of progress monitoring. The results and corresponding conclusions provide evidence to support the external validity of previous simulation-based studies. Implications and future directions for research are discussed.