Research into portfolio assessment ("PA") typically describes teachers' development and implementation of different portfolio models in their respective teaching contexts, however, not much attention is paid to student perceptions of the portfolio approach or its impact on the learning of writing. To this end, this study aims to investigate how two groups of Hong Kong EFL pre-university students (Groups A and B) perceived and responded to two portfolio systems (with each group experiencing one portfolio system either working portfolio or showcase portfolio) in one academic writing course. The case study approach was adopted and data sources included semi-structured interviews, student reflective journals, classroom observations, and analysis of text revisions. Findings indicated that students from the showcase portfolio group (Group B) were less enthusiastic about the effectiveness of PA, and queried whether it could promote autonomy in writing, while the working portfolio group (Group A) was more receptive to the experience, and considered that a feedback-rich environment in the working portfolio system could facilitate writing improvement. The paper concludes with a discussion of how PA can be used to promote self-regulation in the learning of writing. (Contains 10 figures and 7 tables.)