While research has shown the statistical significance of high school grade point averages (HSGPAs) in predicting future academic outcomes, the systems with which HSGPAs are calculated vary drastically across schools. Some schools employ unweighted grades that carry the same point value regardless of the course in which they are earned; other schools use weighting systems that assign greater value to grades earned in honors courses. Due to these inconsistencies, comparison of HSGPAs from different schools is difficult or impossible. We coded 710 transcripts from undergraduate students involved in the Joint Admissions Medical Program in Texas. All grades were standardized on an unweighted 4.0 scale in order to compare the effectiveness of weighted and unweighted HSGPAs. Using multiple regression and multiple logistic regression models, we were able to determine the predictive power of HSGPAs on four outcomes: college grade point average (CGPA), Medical College Admissions Test (MCAT) scores, the likelihood of students taking the MCAT, and the likelihood of students graduating from medical school. Our results demonstrated that unweighted HSGPAs were better predictors of CGPA, but that neither type of HSGPA was a useful predictor for the other outcomes. We recommend discontinuation of the use of weighted HSGPAs in assessing the likelihood of student success in higher education.