Research on problem-based learning (PBL) has tended to focus on the graduate level of education, paying less attention to the primary school level and to what is involved for students during the implementation of PBL. In this paper we take a step towards addressing this need for research by reporting findings from a descriptive, explanatory case study of a one-year PBL intervention in a fifth-grade (ages 10-11) mathematics classroom in Taiwan. We documented and analysed the students' actions to reveal details in their part in the intervention. We compared the students' performances on the five researcher-designed written tests, which were conducted at different points in time. Our findings cast light on the level of the students' participation in PBL. The comparison between the test results shows improvements in students' understanding of the concepts and supports the idea that PBL has positive effects on long-term knowledge retention.