Objectives This study aimed to investigate the relationship between the authenticity of instructional formats and outcome measures within a pre-clerkship clinical reasoning course. Methods We conducted a randomised, prospective, crossover study with Year 2 medical students taking a pre-clerkship clinical reasoning course. Students were randomised to small groups and exposed to three formats of differing instructional authenticity (paper case, DVD presentation, standardised patient [SP] presentation) across three subject areas (abdominal pain, anaemia, polyuria). Three student cohorts were taught using one instructional format per subject area so that each cohort received a different instructional format for each of the three subject areas. Outcome measures (objective structured clinical examination, video quiz, written examination) were selected to determine the effect of each instructional format on the clinical reasoning of students. Results Increasingly authentic instructional formats did not significantly improve clinical reasoning performance across all outcome measures and subject areas. However, the results of the video quiz showed significant differences in the anaemia subject area between students who had been instructed using the paper case and live SP-based formats (scores of 47.4 and 57.6, respectively; p=0.01) and in the abdominal pain subject area, in which students instructed using the DVD format scored higher than students instructed using either the paper case or SP-based formats (scores of 41.6, 34.9 and 31.2, respectively; p=0.002). Conclusions Increasing the authenticity of instructional formats does not appear to significantly improve clinical reasoning performance in a pre-clerkship course. Medical educators should balance increases in authenticity with factors such as cognitive load, subject area and learner experience when designing new instructional formats.