While student loans provide college opportunity for many, undergraduate student debt resulting from typical and heavy borrowing hinders future investments in human capital. Propensity score matching analysis of the NSF's 2003 National Survey of Recent College Graduates demonstrates that debt negatively affects the graduate school enrollment of bachelor's degree holders in STEM fields, where debt is measured by a student's cumulative undergraduate debt relative to the mean debt of his or her baccalaureate graduating cohort. The findings support recent changes in financial aid policy that seek to reduce undergraduate borrowing by increasing means-tested grant aid. (Contains 3 tables and 3 footnotes.)