Portfolio assessment (PA) is promulgated as a useful tool to promote learning through assessment. While the benefits of PA are well documented, there is a lack of empirical research on how students' self-regulation can be effectively fostered in writing classrooms, and how the use of PA can develop students' self-regulated capacities. This multiple case study, which spanned one academic year, explored how teachers can foster self-regulation in elementary students through PA, and the effects of using PA on self-regulation among students. Two teachers and their students from two Primary Six classes in different elementary schools in Hong Kong participated in the study. Data sources included interviews with teachers and students, as well as classroom observations and field notes. The results of the study indicate that portfolios are an empowering activity and contribute to students' development of self-regulated learning. Implications of the study are discussed.