In this study, the authors analyzed data from a sample of thirty-two middle school students from an urban school district in the southeastern United States who used MIT's App Inventor to design, create, and remix mobile apps during an afterschool program for one school year. This paper focuses on computer science learning outcomes as measured by an assessment created by the authors. Findings indicated a linear relation between the number of apps a participant created during the given afterschool program and their level of accuracy on the assessment. Findings and discussion should be of interest to both researchers and practitioners.