Background: Practicing physicians require serious illness communication (SIC) skills to ensure high-quality, humanistic care for patients and families as they face life-changing medical decisions. However, a majority of U.S. medical schools do not require formal training in SIC and fail to provide students deliberate practice before graduation. The Massachusetts Medical Schools' Collaborative was created to ensure that students receive foundational SIC training in undergraduate medical education. This Collaborative developed a curriculum-mapping tool to assess SIC at four medical schools. Objective: We aimed to understand existing educational activities across four medical schools and identify opportunities to build longitudinal, developmentally based curricular threads in SIC. Design: From July 2019 to April 2021, faculty, staff, and medical students assessed current educational activities related to five core competencies in SIC, adapted for students from national competencies for palliative medicine fellows, using a curriculum mapping tool. Measurements: The group selected 23 keywords and collected metrics to describe the timing, instruction and assessment for each school's educational activities. Results: On average, there were only 40 hours of required curricula in SIC over four years. Over 80% of relevant SIC hours occurred as elective experiences, mostly during the postclerkship phase, with limited capacity in these elective experiences. Only one school had SIC educational activities during the clerkship phase when students are developing clinical competencies. Assessment methods focused on student participation, and no school-assessed clinical performance in the clerkship or postclerkship phase. Conclusions: Medical schools are failing to consistently train and ensure basic competency in effective, compassionate SIC. Curriculum mapping allows schools to evaluate their current state on a particular topic such as SIC, ensure proper assessment, and evaluate curricular changes over time. Through the deliberate inclusion of SIC competencies in longitudinal curriculum design, we can fill this training gap and create best practices in undergraduate medical education.