Can process portfolios that support students in goal setting, reflection, self-evaluation and feedback have a positive impact on students' writing self-efficacy? This article presents the findings of a yearlong study conducted in three 4th grade elementary classes in Cyprus where paper-based and web-based portfolios were implemented to help students document their progress. Using a non-equivalent pre-test post-test design it was found that the self-efficacy of students who used portfolios for their essay writing process over an academic year increased significantly (p less than 0.05), compared to control students who did not. The increase of writing self-efficacy corresponded to an increase of writing performance (p less than 0.05). This study also verified the connection between Bandura's self-efficacy theory and portfolio affordances. Instructional implications for portfolio implementation are offered. (Contains 2 figures and 5 tables.)