In England, GCSE (General Certificate of Secondary Education) qualifications offered to students aged 14-16 were recently reformed. For mathematics specifically, the new GCSE aimed to be more demanding, provide greater challenge for the most able students, and support progression to post-16 mathematics. However, there have been concerns that the new GCSE could deter students from further education in the subject and, to date, there has been little research on its impact on participation in and learning of mathematics post-16. This research compared progression to and performance in post-16 mathematics pre- and post-GCSE reform and found that, contrary to fears about reduced uptake, progression generally increased following the reform. In particular, the increase was higher among those who achieved top grades in their mathematics GCSE than for students with just a pass. Performance in post-16 mathematics was, in contrast to teacher expectations, lower post-reform.