For students who show inadequate response to research-based intervention, intensification of intervention using data-based decision making (DBDM) is recommended. There is a paucity of research on upper elementary students related to the efficacy of (a) word reading interventions and (b) DBDM procedures. This randomized controlled trial examined the differential effects of data use at two timepoints to intensify a multisyllabic word reading intervention for Grades 4 and 5 students with or at risk for reading disabilities (RD). Eighty-eight students were randomly assigned to one of three conditions: initial customization of the intervention (IC-only), initial customization with DBDM (IC + DBDM), or a business-as-usual comparison condition. Results indicated that (a) students in both treatment conditions outperformed the comparison condition on multisyllabic word reading and (b) students in the IC + DBDM condition also outperformed comparison students on decoding. Implications, including the use of DBDM in addition to IC-only as well as methods for decision-making within a small-group context, are discussed.