The long run earnings and career potential of individuals are strongly affected by their education. Among college educated individuals, the choice of college major is a very important determinant of labor market outcomes. In most countries men and women exhibit significant differences in this choice which is responsible for a large portion of the gender gap in earnings. In this paper we analyze whether the gender composition of peers (classmates) in high school affects the choice of major and hence long run earning potential. We use a newly collected and unique dataset covering 30,000 Italian students graduated from high school between 1985 and 2005. We exploit the fact that students are assigned to classes whose gender composition, within a school over time, varies exogenously. Moreover we are able to control for family, cohort, teacher and school effects in assessing the effect of peer-gender ratio on outcomes. We find that the gender ratio of peers in high school significantly affected the choice of major. A larger share of same-sex peers increases the probability of choosing majors associated to high earning jobs (Economics/Business, Medicine, Engineering). For women we also find that a large percentage of female high school classmates increases their long run performance in college and their earnings.