In recent decades, several countries have made an effort to increase the enrollment rates and performance of students in science and mathematics by means of mandatory, rigorous course work, which is often referred to as curricular intensification (CI). However, there is a lack of research on intended and unintended effects of CI reforms on achievement and motivation. Using representative data from the National Educational Panel Study, we examined effects of a prototypical CI reform in 1 German state. We compared data from the last student cohort before and the first student cohort after the reform at the end of upper secondary school. There was no statistically significant effect on average achievement. However, we found first evidence for differential effects on English reading and a higher English self-concept of young men after the reform, whereas the reform had a negative effect on young women's math self-concept.