At most medical schools students spend the core clerkship year entirely in clinical settings, geographically dispersed, and assigned to separate teams. Because of the immediacy of experiential learning in the clinical environment, this year is often the highlight of medical school. However, the intensity of the experience and the dispersion of students poses serious challenges to student well being and professional development, and to meeting important educational objectives best taught in the clinical year but difficult to implement in competition with direct patient care. To address these challenges in a way that does not interfere with the clinical experiences, we developed and are implementing three one-week intersessions. These are designated weeks between clerkship rotations when all third-year students are "off rotations" and studying together in an integrative, collaborative and reflective manner. We identified themes for the intersession course from several sources. In response to strong documentation by past students of isolation and insufficient opportunity to reflect on their experiences during the core clerkships we placed a high priority on students gathering together as a class and on professional development. Additionally, based on knowledge gaps identified in the AAMC Graduation Questionnaire and our commitment to integrating basic science teaching into the clinical year, we developed the following five themes: evidence-based medicine, ethics, health systems (quality; resource allocation), advances in science (recent advances that fundamentally shift clinical practice), and professional development. Each intersession week consists of 20 hours of structured contact time; 75% is devoted to small-group learning. The weeks rely heavily on student directed and collaborative learning to complete required readings and assignments. All sessions build on the clinical experiences students have had during clerkships and enhance the students' skills for upcoming rotations. In 1999-2000, we restructured the clinical core year into eight-week modules allowing us to gather the entire class, in between clerkships, for intersessions. We phased in two intersession weeks in 2000-2001 and are implementing three intersession weeks in 2001-2002 (October, February, and June). In the evaluations of the first year's intersessions students valued the opportunity to gather together, to process their clinical experience, and to utilize their clinical experience to drive learning in important, clinically relevant areas that are not consistently taught in the clerkships. Evaluations from the first intersession of the second cycle further underscore the preference for learning experiences that are highly relevant to the clinical year (e.g., practicing efficient search strategies to quickly answer clinical questions, utilizing systematic reviews, discussing ethics cases from the students' experiences) and the benefits of faculty facilitated small-group discussions over lecture time. The advances in science sessions are most effective when they focus on advances in diseases that students are likely to have encountered. In our next phase, we will use Web-based interfaces to collect cases from students on clerkships and to promote discussion of topics in anticipation of the next intersession. As we continue to refine intersessions, our experience so far provides good evidence to support intersessions as a successful curricular innovation.