This descriptive causal-comparative study examined differences between ACT mathematics subtest scores for students taught using an integrated mathematics curriculum (Integrated Math 1-2-3) and students taught using a traditional mathematics curriculum (Algebra 1-Geometry-Algebra 2). The study also controlled for gender and aptitude. The results of this study add to the existing body of research to help school districts make an informed decision about which mathematics curriculum would best meet the needs of their students. The data used in this study were obtained from four high schools in two school districts located in Tennessee. One school district switched from a traditional mathematics curriculum to an integrated mathematics curriculum at the start of the 2012-2013 school year and the second school district transitioned at the start of the 2015-2016 school year. Data were analyzed using Model 3 of the Hayes (2018) PROCESS macro, ANCOVA, and factorial ANOVA. The study found that in three of the four included high schools, a traditional mathematics curriculum was associated with higher achievement on the ACT mathematics subtest than an integrated mathematics curriculum. The results for the fourth high school indicated that student achievement did not necessarily differ depending on the mathematics curriculum employed. The study also found that the moderating effects of gender and aptitude were not significant. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]