There is growing support for making the study of mathematics to the age of 18 years compulsory for all young people in England. This article aims to inform this debate through new insights into historic A-Level Mathematics participation trends. We analyse full-year cohorts of 16-year-old students from the Department for Education's National Pupil Database from 2004 to 2010, a total of just over 4.5 million young people. Using a cohort-tracking approach, we aim to better understand the flow of young people through upper secondary mathematics education. Earlier work identified General Certificate of Secondary Education (GCSE) attainment as the strongest predictor of A-Level Mathematics participation. In this article, we show that the percentage of students progressing to A-Level by GCSE grade has not changed significantly over the period in question, with some exceptions. This implies that the increase in A-Level Mathematics numbers is largely explained by the growing proportion of higher GCSE grades. We discuss the implications for policy that this raises, e.g. the possible impact of making GCSE mathematics more demanding.