AIM: Portfolios are often used as an instrument with which to stimulate students to reflect on their experiences. Research has shown that working with portfolios does not automatically stimulate reflection. In this study we addressed the question: What are the conditions for successful reflective use of portfolios in undergraduate medical education? METHODOLOGY/RESEARCH DESIGN: We designed a portfolio that was aimed at stimulating reflection in early undergraduate medical education, using experiences described in the medical education literature and elsewhere. Conditions for reflective portfolio use were identified through interviews with 13 teachers (mentors), who were experienced in mentoring students in the process of developing their portfolios. The interviews were analysed according to the principles of grounded theory. RESULTS: The conditions for successful reflective use of portfolios that emerged from the interviews fell into 4 categories: coaching; portfolio structure and guidelines; relevant experiences and materials, and summative assessment. According to the mentors, working with a portfolio designed to meet these conditions will stimulate students' reflective abilities. CONCLUSION: This study shows that portfolios are a potentially valuable method of assessing and developing students' reflective skills in undergraduate medical training, provided certain conditions for effective portfolios are recognised and met. Portfolios have a strong potential for enhancing learning and assessment but they are very vulnerable and may easily lead to disappointment. Before implementing portfolios in education, one should first consider whether the necessary conditions can be fulfilled, including an appropriate portfolio structure, an appropriate assessment procedure, the provision of enough new experiences and materials, and sufficient teacher capacity for adequate coaching and assessment. © Blackwell Publishing Ltd 2005.