Previous research suggests that racial and ethnic disparities in postsecondary STEM outcomes are rooted much earlier in the educational pipeline. One possible remedy to these disparities is participation in early STEM enrichment programs. We examine the impact of MESA, which is an early program that targets socioeconomically disadvantaged students, on outcomes that may lead students down the path to STEM. We analyze three waves of restricted nationally-representative data from the High School Longitudinal Study that trace the STEM progress of more than 25,000 students throughout high school and into their postsecondary careers. Propensity score matching models reveal that MESA participation increases students' odds of taking AP STEM courses in high school and their aspirations for declaring a STEM major in college. However, these effects are driven primarily by black and white students, respectively. Latino and Asian students remain largely unaffected. A formal sensitivity analysis concludes that these findings are moderately robust to unobserved confounding. The results are also robust to alternative matching schemes. Collectively, the findings suggest that MESA may improve black students' high school STEM engagement but may have little impact on black and Latino students' STEM outcomes in college.