Retention of students to science, technology, engineering, and mathematics (STEM) major has been studied for four cohorts participating in a summer bridge program supported by the National Science Foundation. Students participated in a 6-week program prior to their first term of enrollment at a research-intensive land grant university. Comparisons to baseline incoming classes from 2009-2011 show gains in retention to major out to the third year for underrepresented minorities (African American, Hispanic, Native American), first-generation college students, and females. Gains for underrepresented minorities and females are statistically significant, whereas those for first-generation students are not. These gains are associated with participation in a 6-week summer bridge program and the resulting improvement in preparation for college coursework, sense of belonging as measured by qualitative surveys, and use of academic support structures.