This study tracked 3,286 students over a five-year period who were enrolled in entry-level biology, chemistry, mathematics, and physics courses offering SI in the fall 1999 to see if they were retained or graduated at a Midwestern doctoral extensive institution and identified which predictor variables (demographic, achievement, and level of SI participation) most significantly predicted student retention or graduation. Chi-square analysis, based on two-way contingency tables indicated that SI participants are retained at higher rates than non-SI participants while having lower mean ACT composite scores and fewer semesters of high school preparation in calculus, chemistry, and physics. Backward stepwise multiple logistic regression analysis was used to determine that the most significant predictor of student retention/graduation was high school rank. Positive predictors for the various disciplines across the five year period included the number of SI sessions attended the number of transfer credits earned, and the number of semesters of high school calculus, chemistry, or physics. Negative predictors of student retention or graduation included Pell Grant eligibility and being a member of ethnic minority group.;The results of this study, in addition to making a significant contribution to literature on retention and SI, also have implications for institutional practice. Specifically, this study provides a model for evaluating SI programs or other academic support programs to demonstrate how the program helps retain students. The findings also may be used to inform institutional leaders, policymakers, and the public about how SI is a useful tool to retain students and encourage the expansion of SI programs to meet the needs of additional learners.