Background: There is little evidence regarding the effects of reasoning visualization decision support aids on the quality of clinicians’ diagnostic decisions. Methods: To address this evidence gap, we conducted a randomized-controlled study involving 30 participating clinicians from three levels of experience: medical students, residents, and physicians. Two interventions designed to improve the diagnostic approach to pulmonary embolism were assessed: a visualization reasoning task model vs. more traditional didactic lecture. All participants were requested to solve paper-based clinical scenarios. The primary outcome of diagnostic pathway concordance (derived as a ratio of the number of correct diagnostic decision steps divided by the ideal number of diagnostic decision steps in diagnostic algorithms) was measured at baseline (5 clinical scenarios) and after either intervention. Results: The mean of diagnostic pathway concordance improved in both study groups: baseline mean= 0.73, post mean (scenarios 1-10) for the decision support group = 0.90 (p < 0.001, 95% CI: 0.08-0.24); baseline mean= 0.70, post mean (scenarios 1-10) for didactic review group = 0.85 (p < 0.001, 95% CI: 0.06-0.2). There was no statistically significant difference between the two study groups, or between the three levels of participants. Conclusions: Clinical decision support aids that are designed with an evidence-based visual reasoning interface can improve clinicians’ diagnostic pathway concordance. To the extent that such decision support aids can feasibly be implemented in clinical settings, while didactic lectures at crucial moments in patient care cannot, they have potential advantages over the latter.