Increasing the number of students who study mathematics once it is no longer compulsory remains a priority for England. A longitudinal cohort from England (1085 students) was surveyed at Years 10 and 12. Students' self-beliefs of ability influenced their GCSE mathematics grades and their intended and actual mathematics subject-choices; the degree of under-confidence or over-confidence related to these self-beliefs was also influential. Additional factors that significantly influenced students' intentions at Year 10 to study mathematics in Year 12 were the advice or pressure to do so, the extrinsic motivation associated with mathematics, their gender and the emotional response associated with doing mathematics. These same factors were also significant influences on students' intentions at Year 12 to study mathematics at university, with the addition of their intrinsic motivation associated with mathematics. Although gender was not a significant influence on GCSE mathematics grades or whether students actually studied A-Level mathematics, boys were associated with higher intentions to study mathematics into Year 12, 13 and university. Additionally, girls were generally more under-confident than boys in their self-beliefs.