Dual enrollment (DE), an arrangement by which high school students take college courses, is becoming increasingly popular as a means of improving high school education. However, there is very little rigorous evidence on its impact on student outcomes. A particular concern in evaluating its effects is the selection bias that arises because more able students are more likely to take DE courses. In this study, I employ a quasi-experimental method to gauge the causal effects of DE on student outcomes. I conduct two regression discontinuity analyses that exploit a statutory mandate in the state of Florida requiring high school students to have a minimum academic standing in order to participate in DE. The first analysis evaluates the effects of DE using GPA as the eligibility criterion. The second analysis evaluates the effects of a particularly challenging and popular DE course, college algebra, using an eligibility criterion that is specific to that course. While the standard regression-discontinuity methods are appropriate for the first analysis, the participation criterion for college algebra is used not only for DE students but also for college students. I therefore employ an extension of the regression-discontinuity method that accounts for sequential treatments. Using data on students from two high school cohorts (2000-01 and 2001-02) in selected Florida districts who were tracked through the summer of 2007, I find no evidence that simply taking a DE course improved marginal students' rates of high school graduation, college enrollment, or college degree attainment. However, for students on the margin of participation in algebra, I find that taking such a challenging DE course had large and significant effects on college enrollment and graduation rates. Appended are: (1) Estimated Discontinuity in Dual Enrollment (Any Course) in 12th Grade by Community College; and (2) Estimated Discontinuity in Participation in Dual Enrollment Algebra by College. (Contains 9 tables, 10 figures and 38 footnotes.)