This study provides significant advances in the understanding and utility of writing assessment for progress monitoring writing instruction. We explored the validity of a new writing tool that asks students in Grades 3 through 8 to read and respond to informational passages. The written response is then scored for writing fluency. Results indicated that students' writing fluency facilitated their writing quality and predicted 70% to 95% of the variance in writing achievement among students in middle school and 31% of the variance in Grade 3. To further validate the use of passages in progress monitoring, we used a rigorous method (latent variable equating) to remove the measurement error due to different passages. Considerations for instructional decisions based on writing assessment, as well as limitations of the study, are discussed.