In the literature, there is a general concern that a less number of students choose to study science, technology, engineering or mathematics (STEM). This paper presents results from a Danish longitudinal study which examines students' choice of whether or not to continue studying STEM after upper-secondary school. In particular, this study focuses on students who held an STEM subject as one of their favourite subjects at secondary educational level, but who chose not to study STEM at the tertiary level. This paper explores how students' perceptions of STEM relate to their identity work. The data used, primarily consist of interviews with 38 students at the end of upper-secondary school. The analysis explores the students' expectations of what higher education STEM might be like. These expectations are contrasted with the first-year experiences of 18 of the 38 students who eventually entered a higher education STEM programme. The results show that the students who did not choose STEM, perceived STEM as stable, rigid and fixed, and, hence, too narrow a platform for developing and constructing desirable identities. The experiences of those students who actually entered a STEM programme turned out to be similar to these expectations. However, many choosers would also prefer their studies as less rigid and fixed. If the institutions could adjust to the form and content of the courses, it might both meet the interests of choosers and non-choosers and thereby both increase recruitment and retention at STEM higher education programmes.