One recommended way to intensify intervention, particularly for struggling readers with the most severe difficulties, is intervention intensification using student data—a process referred to as data-based individualization (DBI; Deno & Mirkin, 1977; National Center on Intensive Intervention [NCII], 2013). To date, there is a dearth of research on word reading interventions that target 4ᵗʰ and 5ᵗʰ grade students (Wanzek, Wexler, Vaughn, & Ciullo, 2010), as well as reading interventions that utilize DBI to intensify such interventions (Filderman, Toste, Didion, Peng, & Clemens, 2018). As such, this randomized controlled trial explored the use of DBI for 4ᵗʰ and 5ᵗʰ grade struggling readers within the context of a research-based multisyllabic word reading intervention (Toste, Capin, Vaughn, Roberts, & Kearns, 2017; Toste, Capin, Williams, Cho, & Vaughn, 2019). In addition to adding to the literature on this understudied population, I also evaluated whether the use of data to customize instruction at the beginning of intervention is enough to improve results of struggling readers, or whether additional adjustment using progress monitoring with explicit decision-making rules improves results above and beyond initial customization of intervention protocol. Accordingly, I compared two treatment conditions to a business-as-usual condition. One treatment condition received initial customization of intervention, with adjustments made to the amount of time students spent in each of the instructional components based on their initial decoding abilities. The second treatment condition received the same customization, but at the mid-point of the study, ongoing curriculum-based measurement and specific subskill mastery measurement was used to evaluate student response, with instruction individualized for students who demonstrated inadequate response. Results indicated that students in both treatment conditions outperformed the comparison condition on multisyllabic word reading, and that students in the DBI condition also outperformed comparison students on decoding. Both treatment conditions performed worse than the comparison on a test of sentence-level comprehension efficiency. I conclude with a discussion of the potential reasons for these findings, as well as implications and future directions for research and practice.