Background/Context: Taking even one Advanced Placement (AP), International Baccalaureate (IB), dual enrollment, or other college-preparatory course is positively associated with student outcomes including test scores; high school graduation; and college matriculation, major choice, and completion (Adelman, 2006; An & Taylor, 2019; Author, 2017, 2020; Avery et al., 2018; Conger et al., 2021; Jackson, 2014). Benefits are especially large for students from historically underrepresented backgrounds (An, 2013a, 2013b; Lee et al., 2022; Long et al., 2012). However, large inequities in advanced course participation persist, despite high postsecondary aspirations among students of color and students from low-income backgrounds (Schneider & Saw, 2016). These inequities are substantially driven by within-school inequities in advanced course enrollments (Kolluri, 2018; Patrick et al., 2020; Xu et al., 2021). Purpose/Research Question: We examine whether an automatic enrollment policy in Washington state increases access to advanced courses for students who have historically been underrepresented in these courses. Setting: Academic Acceleration was first implemented in 2010-11 by an urban district serving about 20,000 students south of Seattle. Two years later, the Washington state legislature passed a law encouraging all districts to adopt the policy. Between 2013-14 and 2015-16, 72 of Washington's 263 districts with at least one high school adopted the policy. In 2019, the legislature updated the policy and mandated that all districts adopt Academic Acceleration by 2021-22. Intervention/Program/Practice: Academic Acceleration attempts to increase equity in advanced course enrollment in two ways. First, it requires high schools to automatically identify students as eligible for advanced courses if they met the state standard on statewide assessments, rather than relying on teacher, counselor, student, or parent input. Second, it makes advanced courses opt out rather than opt in for qualified students by requiring schools to automatically enroll all qualified students into the next highest level of coursework in the same or related subject(s). Research Design: We employed a difference-in-differences model to examine changes in course enrollment in policy-implementing districts compared to changes in comparison districts. We define advanced courses as AP, IB, Cambridge International, Running Start (a dual enrollment program at community colleges), or College in the High School (a dual credit program in high schools). We also examined GPA, enrollment in a subsequent advanced course in grade 12 (for grade 11 students), and high school graduation. We estimated separate models by advanced ELA or social studies courses, advanced mathematics courses, and any advanced course regardless of subject area for students in grades 11 and 12. We estimated differential outcomes for qualified students and students in the URM and FRPL groups using two- and three-way interactions. We used linear probability models, implemented two-way fixed effects, and clustered standard errors at the district level (the level of policy variation). Data Collection and Analysis: We used administrative, student-level data from the Washington Office of Superintendent of Public Instruction for all 11th- and 12th-grade students in Washington public schools from 2010-11 through 2018-19. 263 Washington district have at least one high school. We excluded two very early adopting districts and 15 late adopting districts for insufficient pre- or post-policy data. We also excluded six districts with no pre-period high school enrollment. We generated a matched analytic sample of 72 districts in our three implementation cohorts and 168 comparison districts that did not adopt Academic Acceleration before the 2019 legislative update. The prepolicy implementation trends fail the parallel assumptions test for the 2015 and 2016 cohorts. There is an upward trend in the rate of advanced course taking in treatment districts before implementation but not comparison districts, and the formal test (De Chaisemartin & d'Haultfoeuille, 2020) rejects the null hypothesis of parallel trends (p = 0.03). Propensity score matching did not resolve the violations As a result, our findings should be viewed as descriptive, not causal. Findings/Results: Academic Acceleration was associated with a 5.3 percentage point increase in the probability of enrollment in any advanced course for all students, regardless of qualified status, and a 4.8 percentage point increase in the probability of advanced mathematics courses enrollment for qualified students. It also was associated with a 5.1 percentage point increase in qualified students' chances of taking any advanced course in the 2017 cohort. In most cases, Academic Acceleration was associated with a similar change in the odds of advanced course enrollment for qualified URM or FRPL-eligible students and qualified non-URM or non-FRPL-eligible students. In the 2017 cohort, however, qualified non-URM and non-FRPL-eligible students in policy-implementing districts experienced 7.4 and 6.5 percentage point larger increases in the probability of taking advanced courses, respectively, than qualified URM and FRPL-eligible students. Across cohorts, policy implementation was associated with a 2 percentage point larger increase in ELA/social studies enrollment, and with a 3.1 and 3.8 percentage point larger increase in mathematics enrollment, for URM and FRPL-eligible students, respectively, compared with non-URM and non-FRPL-eligible students. Taken together, these changes translate into narrowed enrollment differences by URM and FRPL status. Notably, treated districts experienced increases in advanced course enrollment for URM groups relative to non-URM groups regardless of qualified status. Students' performance in courses targeted by the policy and other courses remained similar or showed small increases from pre- to post-policy. The policy also was associated with a 4.6 percentage-point higher probability of enrolling in another advanced math course in 12th grade (for 11th graders), and no difference in the probability of on-time high school graduation. Conclusions: Increases in the probability of advanced mathematics course enrollment among qualified students in treatment districts suggest that Academic Acceleration may have identified and enrolled qualified students who would not have enrolled before policy implementation. Interestingly, the increases in any advanced course enrollment for all students (qualified and not qualified) suggest that districts with Academic Acceleration policies also may have broadened participation by expanding their eligibility criteria beyond standardized test scores or encouraging students to try an advanced course outside the core subjects. Findings beyond enrollment indicate the policy increased access to advanced courses without decreasing student performance. However, demographic inequities remained, especially in advanced mathematics.