PURPOSE: To determine the impact of increasing medical school class size on the internal medicine clerkship. METHOD: In 2006, the Clerkship Directors in Internal Medicine surveyed its 110 institutional members to discover whether their medical school class size had increased (or would increase) and the impact of increasing class size on number of students per teaching site, number of clerkship sites needed, and resources needed. Respondents rated their agreement or disagreement with statements about increasing class size, and they provided free-text responses. Analyses included descriptive statistics and qualitative analysis. RESULTS: Response rate was 76% (83/110). In the three years preceding the survey, one medical school decreased class size; 43% increased by a mean of 14 students per year (10%). Most respondents (51%) expected class size to increase by a mean of 17 students per year (12%) through 2009; none expected class size to decrease.Increasing class size by 15% would mean adding 3.7 (standard deviation = 2.2) students per inpatient site, 2.9 (2.9) new inpatient sites, 3.0 (2.2) students per ambulatory site, and 4.9 (5.5) ambulatory sites.Respondents disagreed with the questionnaire statements that they would have more resources, teachers, and protected time; they agreed with statements that recruiting teachers would be harder as class size increases. Free-text responses to the challenges of increasing class size revealed two themes: strain on resources (space, time, faculty), and the impact on the educational experience. CONCLUSIONS: Internal medicine clerkship directors believe increasing medical school class size will dramatically increase resources needed during clerkships and may adversely impact education.