More women are now entering male-dominated fields, yet, science, technology, engineering, and mathematics (STEM) remain dominated by men. We examined the association between boys' and girls' STEM choices after secondary education and friends' gender norms, and whether pressure to conform to traditional gender norms differs depending on the gender composition of the friend group. Drawing on 3 waves of longitudinal data (N = 744) from the Netherlands, our sample consists of adolescents in STEM trajectories in secondary education. Their retention in STEM after secondary education gives us a better understanding of gender-specific "leakage" from the STEM pipeline. We found that girls' likelihood of choosing STEM decreased drastically when friends had more traditional gender norms. Friends with traditional gender norms had less effect on boys. Nonetheless, boys with only same-sex friends were more likely to enter STEM. Our findings indicate that an environment with gender-normative ideas pushes girls out of the STEM pipeline.