CONTEXT:Diagnostic efficiency is important in daily clinical practice as doctors have to face problems within a limited time frame. To foster the clinical reasoning of students is a major challenge in medical education research. Little is known about students' diagnostic efficiency. On the basis of current theories, scaffolds for case representation (statement of the case as far as it is summarised in the mind) could be a promising approach to make the diagnostic reasoning of intermediate medical students more efficient. METHODS:Clinical case processing of 88 medical students in their fourth and fifth years was analysed in a randomised, controlled laboratory study. Cases dealing with dyspnoea were provided in an electronic learning environment (CASUS). Students could freely choose the time, amount and sequence of clinical information. During the learning phase the intervention group was asked to write down case representation summaries while working on the cases. In the assessment phase diagnostic efficiency was operationalised as the number of correct diagnoses divided by the time spent on diagnosing. RESULTS:Diagnostic efficiency was significantly improved by the representation scaffolding (M = 0.12 [SD = 0.07], M = 0.09 [SD = 0.06] correct cases/time, p = 0.045), whereas accuracy remained unchanged (M = 2.28 [SD = 1.10], M = 2.09 [SD = 1.08], p = 0.52). Both groups screened the same amount of clinical information, but the scaffolding group did this faster (M = 20.8 minutes [SD = 7.15], M = 24.6 minutes [SD = 7.42], p = 0.01; Cohen's d = 0.5). CONCLUSION:Diagnostic efficiency is an important outcome variable in clinical reasoning research as it corresponds to workplace challenges. Scaffolding for case representations significantly improved the diagnostic efficiency of fourth and fifth-year medical students, most likely because of a more targeted screening of the available information.