Building on a previous randomized experiment of the impact of Early Colleges (ECs) (Berger et al., 2013), this follow-up study assessed longer-term impacts of ECs on students' postsecondary outcomes 6 years after expected high school graduation. It also explored the extent to which students' high school experiences mediate EC impacts. Specifically, this study addressed three research questions: (1) Did EC students have better postsecondary outcomes (i.e., college enrollment and degree attainment) than control students? (2) Did the impacts of ECs vary by student background characteristics (i.e., gender, race/ethnicity, low-income status, and prior mathematics and English language arts [ELA] achievement)? and (3) Were the impacts of ECs mediated by students' high school experiences (i.e., college credit accrual during high school, instructional rigor, college-going culture, and student supports)? To answer these questions for the follow-up study, the authors analyzed 4 more years of postsecondary outcome data from the StudentTracker Service at the National Student Clearinghouse for students participating in the EC admission lotteries that were the basis of the previous impact study. They also analyzed data on student background characteristics from administrative records and data on high school experiences from a student survey administered in the previous impact study 5 or 6 years after students entered the ninth grade. [To view the earlier report, "Early College, Early Success: Early College High School Initiative Impact Study," see ED577243.]