In this study, we examined students' mathematical understanding and retention in a problem-based learning (PBL) classroom. The participants were 48 Grade 10 students, and the data were collected in December of 2016. After the end of the PBL lessons, the Mathematical Understanding of Function Test (MUFT) and the Retention Test (RT) were administered. The findings showed that most students demonstrated mathematical understanding of functions in all components and more than 50 percent of the students could pass both tests by the overall mean scores. Moreover, the overall mean difference between the MUFT and the RT was low, which means that the students had retention.