This action research study examined the effect of Read 180, a research-based reading intervention program, on the reading achievement among struggling readers in Grades 9-11 as measured by reading clusters on the Language Arts Literacy portion of the High School Proficiency Assessment, final English grades, and Lexile scores. Struggling readers entering high school often fail to meet stringent literacy demands because they lack mastery of the five main components of effective reading. Although many reading intervention programs exist at the elementary level, few are available in high schools; however, increased pressure for improved student performance on state assessments has caused school districts to explore various programs at the high school level. This study contributes to the literature on such programs by investigating the effects of Read 180 on students' reading achievement. The final grades and assessment scores of three cohorts of ninth-grade students (2007-2008, 2008-2009, and 2009-2010; N = 134) were examined in a matched-pair design, matching students in Read 180 (treatment group) with students in the traditional English 9 course (control group). The scores of the Language Arts Literacy portion of the eighth-grade state assessment determined students' placement. Analyses of variance indicated Read 180 participants significantly outperformed nonparticipants on final English 9 grades. Additionally, a t test indicated Read 180 participants from the 2009-2010 cohort significantly increased their reading achievement according to Lexile scores. Significant results occurred during the school year students participated in Read 180. No other statistically significant differences on the reading achievement between the two groups were evident after students exited Read 180. [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.]