Advanced placement (AP) and concurrent enrollment (CE) provide high school students with rigorous coursework and possible college credit. Theoretical modeling predicted students would substitute CE for AP courses conditional on their probability of earning university credit, passing AP tests, and college selectivity despite CE costing more than AP. In the current study, CE costs to families drop to zero and students should be expected to maximize substitution. This study uses multiple years of school-level data from Colorado, a state with a growing CE sector to test substitution effects. Using a school fixed-effect Poisson regression of the most commonly taken AP exams, results indicate limited evidence of widespread substitution. The continued preference for AP may increase costs to families and reduce potential college credits.