Problem-based learning (PBL) has been introduced into the undergraduate curricula in most Canadian medical schools. In the University of Western Ontario model, we set aside 20% of time to PBL in the first two years cB the programme and the rest for didactic methods. The objective of this article is to discuss 6 years of our experience with the model and the process and problems of introducing PBL into a traditional curriculum. The importance, and methods for developing good cases which are multidisciplinary and 'three-dimensional' (biological, behavioural/psychosocial and population issues) are emphasized. The challenges have been to: integrate the underlying teaching/learning values of two different methods, develop a valid and reliable examination/evaluation procedure, and keep the faculty interested and continually improving their skills. Students have welcomed the break from lectures and the challenge to explore in-depth areas of special interest. The most positive outcomes of this curricular change have been an increased interest in medical education reform in general, and enhanced recognition of teaching in relation to appointments and promotions.