BACKGROUND: As case-based methods replaced lectures in a surgical clerkship, the influences of case structure and prior experience on learning were investigated.METHODS: Early and late third-year students randomly received different cases. ''Structured'' cases had data presented and summarized. ''Unstructured'' cases required questions to faculty for information. Multiple choice tests and differential diagnosis activities were administered. An attitudinal questionnaire gauged student perceptions.RESULTS: In both multiple choice and differential diagnosis activities, the late rotation, ''unstructured'' group scored higher than the ''structured'' group. Conversely, the early rotation, ''unstructured'' group scored lower than the ''structured'' group. Combined, rotation, and structure significantly affected both multiple choice and differential diagnosis activities (ANOVA, P less than or equal to 0.02). Early rotation, ''unstructured'' students described a more enjoyable experience, despite lower evaluation scores.CONCLUSIONS: Surgical clerkship case-based learning is profoundly affected by case structure and prior clinical experience. Case-based curriculum should be tailored to accommodate these interactions.