Many recent high school graduates remain inadequately prepared for college and are required to enroll in remedial or developmental education courses in mathematics or English upon enrollment in college. High rates of college remediation are associated with lower progression and college completion rates. To address this problem, some states, districts, and individual high schools have introduced "transition courses" to prepare students for college-level math and English coursework. Transition courses are typically offered to high school seniors who have been assessed as being underprepared for college math or English. This study uses a regression discontinuity design to estimate the effect of participation in a mathematics transition course on college-level math outcomes in West Virginia for the 2011-12 and 2012-13 high school senior cohorts. Our findings suggest that, among students who scored very close to the cutoff score on an assessment used to decide what students took the course, the math transition course had no statistically significant effect on improving college readiness (as measured by exemption from remedial education upon college entry due to a passing score on a placement test) and in fact had a negative impact on students' likelihood of passing a college gatekeeper math course. Possible explanations for these outcomes include that (1) the transition course may have displaced traditional senior-year courses that were in practice more rigorous than the transition course or that provided positive impacts from inclusion of higher performing peers, and that (2) the transition course curricula may not have been well aligned to the skills required for success on the COMPASS placement test. Most students who took the transition course did not pass the COMPASS, which was taken at the conclusion of the course. The specific math course studied is no longer offered; math transition courses in West Virginia now use a different curriculum.