Background/Context: Sometimes students with the academic credentials to attend more selective colleges enroll in a less selective institution or do not enroll at all, a phenomenon known as undermatching (Bowen et al., 2009; Roderick et al., 2011; Smith et al., 2013; Rodriguez, 2015). Undermatching is associated with lower graduation rates and longer degree completion times at four-year colleges (Bowen et al., 2009). Undermatching disproportionately affects low-income students and students of color (Lowry, 2017), particularly students from these groups with high levels of achievement in high school (Cortes & Lincove, 2019). Previous research has established definitions for undermatching using high school GPA and test scores (Bowen et al., 2009; Roderick et al., 2011; Rodriguez, 2015). This study applies such a framework to the early college high school (ECHS) setting. This paper combines research about the ECHS model and postsecondary undermatching using an RCT. We find that ECHS students are less likely to undermatch at post-high school institutions than control students. These findings are robust to various model specifications and undermatch definitions. Research Questions: This paper analyzes the impact of attending an ECHS on undermatching outcomes, answering the following research questions: What is the impact of attending an ECHS on: (1) Undermatching at a student's first post-high school postsecondary institution; (2) Extent of undermatching at a student's first post-high school postsecondary institution (3) Undermatching for all post-high school semesters of enrollment; and (4) Proportion of semesters enrolled at undermatched institutions? (Completed by May 2022) To what extent do these outcomes differ for: (1) Economically disadvantaged; (2) First-generation; and (3) Students of color? Setting/Population: We use data from a longitudinal North Carolina RCT study (treatment students attended ECHS, control students attended business-as-usual high school). The analytic sample contains 1,993 students (48% economically disadvantaged, 35% students of color, 38% first-generation college) entering ninth grade in 2009 and 2010. Intervention: The ECHS merges high school and college experiences, allowing students to earn an associate degree concurrently with a high school diploma. Students receive dual enrollment opportunities and supports navigating the postsecondary environment, focused on underserved populations (first-generation, economically disadvantaged, students of color) (Edmunds et al., 2017). Analyses have shown positive impacts of ECHS on earning college credits, enrolling in postsecondary education (Edmunds et al., 2017), and earning postsecondary credentials (Edmunds et al., 2020). Research Design: To determine undermatching incidences, we compared the postsecondary institutions students attended and the highest level of institution a student could likely attend predicted by their high school performance (GPA and SAT/ACT score). Consistent with previous studies on undermatching (Bowen et al., 2009; Roderick et al., 2011; Smith et al., 2013; Rodriguez, 2015), we used Barron's selectivity codes to determine the "level" of each institution. We then used National Student Clearinghouse data to categorize the selectivity level of each institution attended by sample students. To classify students as undermatched, we compared the level of the institution of enrollment to the level of matched institution determined by the "predicted probability" taxonomy in Figure 3 (Rodriguez, 2015). Students whose institutions of attendance were at lower levels than their "matched" level or who did not enroll in college after high school were undermatched. All other students were "not" undermatched. We used a regression model for each outcome with treatment status, demographic covariates, lottery indicators, and weights determined by the probability of treatment assignment within each lottery. We estimated clustered standard errors by high school. Data Collection/Analysis: We conducted lotteries at 14 oversubscribed ECHS for the ninth-grade classes of 2009 and 2010 (26 cohorts). Treatment students were offered admission at an ECHS; control students attended a business-as-usual high school. Additional data include unweighted high school GPA and ACT/SAT scores, National Student Clearinghouse (NSC) postsecondary enrollment by semester, and college selectivity (Barron's Selectivity Index). Our total attrition of 16.5% and differential attrition of 3.5% allow us to maintain an experimental contrast. Findings: More ECHS students enrolled in selective institutions in their first post-HS semester than control students. Conversely, more control students enrolled in a two-year college than ECHS students (Figure 1). Treatment and control students "matched" to postsecondary institutions at similar levels (Figure 2). Treatment students were slightly more likely than control students to be matched to 'selective' vs. 'very selective' institutions. Our regression analysis (Table 1) determined found that ECHS students were: (1) 1 percentage points more likely to attend a 'Selective' or higher institution; (2) 0 percentage points less likely to undermatch at their first post-high school; (3) 4 percentage points less likely to severely undermatch (two or more levels below); (4) 2 percentage points less likely to 'always undermatch'; and (5) Undermatched for 9.4 percentage points fewer semesters. We sought to estimate the treatment-control contrast rather than focus on undermatching point estimates. However, as other scholars note (Bastedo & Flaster, 2014), undermatching analysis requires many assumptions. To test the robustness of our results, we ran sensitivity analyses using different estimations of college match and moving students within an error margin between two matching classifications (Figure 4) to the other classification group, summarized in Table 2. Conclusions: Early college students were less likely to undermatch at colleges they attended after high school, driven by differences in selective vs. two-year institution enrollment. These results were robust to various definitions of undermatch and sensitivity analysis at the margins of our classification model. One limitation is the method we used to categorize the "matched" institution for each student. We relied on the outputs of such models to classify students into discrete categories of high school GPA and SAT/ACT score. Future research could combine our sample with a national longitudinal data set to include a more robust predictive model of college match. However, we believe our study confronts this limitation through our sensitivity analysis and by answering questions of experimental contrast rather than point estimates. The results provide several opportunities for additional research on undermatching for students attending early college high schools. We plan the following additional analyses: (1) Exploring underlying mechanisms to explain why ECHS undermatch at lower rates; (2) Linking undermatch status with four-year degree outcomes; and (3) Using NSC data to understand transfer patterns for sample students attending two or more colleges.