Despite its promise to help teachers meet the needs of diverse learners, district usage of Achieve3000® among most students with access to it remains far below the optimal level necessary for them to realize literacy growth and college and career readiness. Only a tiny minority of students (402 out of about 111,000) seem to be engaging Achieve3000 with fidelity, where fidelity is defined as the number of Achieve3000 activities completed over the course of the academic year (i.e., 40 to 80 or more), and an average first-try score (AFTS) earned on Achieve3000 assessments (75% or greater). This study examined how the minority using Achieve3000 with fidelity fared with regard to their Lexile® growth and college and career readiness relative to the majority using Achieve3000 without fidelity. Results show that fidelity of use of Achieve3000 is, net of student characteristics, associated with greater Lexile growth. For two students with an AFTS of less than 75 percent, the one completing 40 activities for the year would gain about 14 Lexile points more than the one who completed only one activity. Likewise, for two students with an AFTS of 75 percent or greater, the one completing 40 activities over the year would gain about 67 more Lexile points than the one who completed only one activity. Fidelity of use of Achieve3000 is also related to higher odds of college and career readiness at the end of the school year, controlling for readiness at the beginning of the year and other student characteristics. For students with an AFTS less than 75 percent, the odds of being college and career ready decrease by about one percent for each Achieve3000 activity completed. For students with an AFTS of 75 percent or greater, the odds increase by four percent. [Title varies: "Assessing the Relationship between Fidelity of Use of Achieve3000 and Student Lexile Growth and College and Career Readiness."]