Elementary teachers’ understanding of mathematics as required for effective classroom teaching is an area of intense research interest. Our research examined the differences in mathematical understanding between pre-service teachers who had been exposed to a reformed mathematics curriculum for up to 8 years during their elementary and secondary education (n = 77) and those who had not had such exposure in school (n = 51). Examination of responses related to integer subtraction and fraction division questions on a paper-and-pencil survey revealed little distinction between the two participant groups. Participants in both groups did not provide any conceptual justification or model to support the operations they were able to perform and resorted to simply restating rules by way of explanation. This inability to think conceptually has implications for in- and pre-service teacher education.