Insufficient student interest in science, technology, engineering, and mathematics (STEM) careers has been identified as a pressing issue by educators and education policy makers. This retrospective cohort study examined one promising approach to reach and inspire students early on: college- and university- run high school STEM summer programs. Data were collected from 27 colleges and universities participating in the National Science Foundation's STEM Talent Expansion Program. We examined the impact of high school STEM summer program participation on end of high school career aspirations among a sample of 845 program participants and 15,002 controls. The study employed logistic regression modeling with propensity weighting to address differences in group characteristics to model the impact of programs. Results showed that students who participated in a program had 1.4 times the odds of wanting to pursue a STEM career, controlling for background characteristics. A closer look at program design revealed that students experiencing the real-world relevance of STEM had 1.8 times the odds of aspiring to STEM careers at the end of high school compared with controls. Findings suggest that scaling up STEM summer programs and carefully designing programs to show the real-life relevance of STEM may be an effective strategy to curtail pipeline attrition and to inspire more students to pursue STEM careers.