Curriculum-based measurement (CBM) is an alternative to traditional assessment techniques. Technical work has begun to identify CBM writing indices that are psychometrically sound for monitoring older students' writing proficiency. This study examined the predictive validity of CBM writing indices in a sample of 447 eighth-grade students. Regression analyses revealed that simple fluency measures were not adequate for assessing secondary students' writing. A more complex fluency measure, the number of correct punctuation marks, and an accuracy-based measure, the percentage of correct word sequences, were the best predictors of a written expression test for eighth-grade students. However, overall results of the current study provided only limited support for the use of CBM to assess writing skill at the secondary level. (Contains 3 tables.)