This paper presents evidence that women and men benefit from having a higher percentage of female peers in post-secondary vocational STEM programs. I use idiosyncratic variation in gender composition across cohorts within majors within branches (campuses) for identification. Having a higher percentage of female peers positively affects students in STEM majors, decreasing women's dropout rates and increasing GPA. The peer effect seems to be mediated by the gender of the instructors: as female students have fewer female instructors, the effect of having more female peers intensifies. For men, a higher percentage of female peers reduces dropouts and increases GPA to a lesser extent, suggesting that policies that increase the representation of women need not entail a trade-off for male STEM students.