Most students with disabilities have reading difficulties and perform significantly lower than their peers without disabilities on low- and high-stakes reading achievement tests. Out of 23 studies on the supplemental Lexia Core5 Reading computer-assisted instruction (CAI) program intervention, 21 studies showed positive reading outcomes for students in the treatment groups, but only two focused on students with disabilities. The problem addressed by this study was the scant evidence demonstrating the effect of the supplemental Lexia Core5 Reading CAI intervention on the reading performance of elementary school students with mild or high incidence disabilities. The purpose of this quantitative causal-comparative study was to determine the effect of the Lexia Core5 Reading CAI intervention for elementary school students with disabilities who used Lexia Core5 Reading as a supplement to an English Language Arts (ELA) curriculum compared with students with disabilities who did not. Framing this study was the Simple View of Reading (SVR) and constructivism theories, including Vygotsky's zone of proximal development (ZPD). Georgia Milestones Assessment System (GMAS) ELA End-of-Grade (EOG) assessment aggregated data from 18 primarily Title I Georgia schools in the Atlanta metro area (N = 613) were analyzed using a Chi-square test of independence. The results were that significant differences were found for all students and within third through fifth grades. The intervention group had a higher proportion of students in the higher or "Proficient"/"Distinguished" and "Developing" levels and lower proportions in the lowest or "Beginning" level when compared to the control/comparison group. A future causal-comparative/ex post facto with pretest-posttest data study on students with disabilities and the Lexia Core5 Reading CAI program intervention is recommended because this type of research is still scant in the published scholarly peer-reviewed literature. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]