The number of students taking high school computer science classes is growing. Increasingly, these students are learning with graphical, block-based programming environments either in place of or prior to traditional text-based programming languages. Despite their growing use in formal settings, relatively little empirical work has been done to understand the impacts of using block-based programming environments in high school classrooms. In this article, we present the results of a 5-week, quasi-experimental study comparing isomorphic block-based and text-based programming environments in an introductory high school programming class. The findings from this study show students in both conditions improved their scores between pre- and postassessments; however, students in the blocks condition showed greater learning gains and a higher level of interest in future computing courses. Students in the text condition viewed their programming experience as more similar to what professional programmers do and as more effective at improving their programming ability. No difference was found between students in the two conditions with respect to confidence or enjoyment. The implications of these findings with respect to pedagogy and design are discussed, along with directions for future work.